Add docs for cross-train command for both luis and qnamaker

packages/luis/docs/using-cross-train-command.md

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+# `luis:cross-train`
+Cross-train writes out updated .lu files that contain cross-trained `_interruption` as well as `DeferToRecognizer` intent. 
+
+Cross-train is required to use the [CrossTrainedRecognizerSet][1] available as part of [adaptive dialogs][2] as well as if you are looking or CI/CD support for [Composer][3] generated bots.
+
+## What is cross-train? 
+With [adaptive dialog][2], you now have the ability to use one [LUIS.ai][4] application per dialog. Cross train command supports this by adding an `_Interruption` intent for each child LUIS application for your bot that has the trigger utterances from all other siblings to that child. 
+
+<img src="./cross-train.png" />
+
+In cases where you are taking advantage of [CrossTrainedRecognizerSet][1], cross train command also sets up a `DeferToRecognizer_QnA_DialogName` with questions from [.qna files][5] automatically pulled in for any dialog.
+
+## Usage
+
+```bash
+> bf luis:cross-train --in <source .lu files> --out <output folder> --config <cross-train configuration>
+```
+
+A cross-train configuration is necessary for the command to work. Here is an example cross-train configuration 
+
+```jsonc
+{
+    // list each .lu file including variations per lang x locale. 
+    // Lang x locale is denoted using 4 letter code. e.g. it-it, fr-fr
+    // Paths can either be absolute (full) paths or paths relative to this config file.
+    "./RootDialog/RootDialog.lu": {
+        // indicate if this is an .lu file for a root dialog.
+        "rootDialog": true,
+        // list of trigers within that dialog
+        "triggers": {
+            // Key is name of intent within the .lu file (in this case RootDialog.lu) 
+            // Value is the path to the child dialog's .lu file.
+            "AddItem": "./AddToDoDialog/AddToDoDialog.lu",
+            "DeleteItem": "./DeleteToDoDialog/DeleteToDoDialog.lu",
+            "ViewItem": "./ViewToDoDialog/ViewToDoDialog.lu",
+            "GetUserProfile": "./GetUserProfileDialog/GetUserProfileDialog.lu"
+        }
+    }
+}
+```
+
+## Related commands
+You might find these additional commands relevant/ helpful - 
+
+- [bf luis:build][6]
+- [bf qnamaker:cross-train][7]
+- [bf qnamaker:build][8]
+
+[1]:https://docs.microsoft.com/en-us/azure/bot-service/bot-builder-concept-adaptive-dialog-recognizers?view=azure-bot-service-4.0#cross-trained-recognizer-set
+[2]:https://aka.ms/adaptive-dialogs
+[3]:https://aka.ms/composer
+[4]:https://luis.ai
+[5]:https://aka.ms/qna-file-format
+[6]:./using-luis-build.md
+[7]:../../qnamaker/docs/using-cross-train-command.md
+[8]:../../qnamaker/using-qnamaker-build.md

packages/qnamaker/docs/using-cross-train-command.md

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+# `qnamaker:cross-train`
+Cross-train writes out updated .qna files that contain cross-trained `DeferToRecognizer` QnA pair. 
+
+Cross-train is required to use the [CrossTrainedRecognizerSet][1] available as part of [adaptive dialogs][2] as well as if you are looking or CI/CD support for [Composer][3] generated bots.
+
+## What is cross-train? 
+With [adaptive dialog][2], you now have the ability to author one or more [.qna file][5] file per dialog. Cross train command supports this by analyzing your cross-train configuration and does the following: 
+- Automatically adds a `dialogName = <dialogName>` meta data filter to each QnA pair
+- Adds one or QnA pairs that pull utterances from .lu content (if any) with answer `intent=DeferToRecognizer_LUIS_DialogName`. 
+
+Both these are necessary for cross-trained recognizer to work. 
+
+<img src="./cross-train.png" />
+
+In cases where you are taking advantage of [CrossTrainedRecognizerSet][1], cross train command also sets up a `_interruption` intent with cross trained uttrances for your interruptions. See [here][10] to learn more. 
+
+## Usage
+
+```bash
+> bf qnamaker:cross-train --in <source .lu files> --out <output folder> --config <cross-train configuration>
+```
+
+A cross-train configuration is necessary for the command to work. Here is an example cross-train configuration 
+
+```jsonc
+{
+    // list each .lu file including variations per lang x locale. 
+    // Lang x locale is denoted using 4 letter code. e.g. it-it, fr-fr
+    // Paths can either be absolute (full) paths or paths relative to this config file.
+    "./RootDialog/RootDialog.lu": {
+        // indicate if this is an .lu file for a root dialog.
+        "rootDialog": true,
+        // list of trigers within that dialog
+        "triggers": {
+            // Key is name of intent within the .lu file (in this case RootDialog.lu) 
+            // Value is the path to the child dialog's .lu file.
+            "AddItem": "./AddToDoDialog/AddToDoDialog.lu",
+            "DeleteItem": "./DeleteToDoDialog/DeleteToDoDialog.lu",
+            "ViewItem": "./ViewToDoDialog/ViewToDoDialog.lu",
+            "GetUserProfile": "./GetUserProfileDialog/GetUserProfileDialog.lu"
+        }
+    }
+}
+```
+
+## Related commands
+You might find these additional commands relevant/ helpful - 
+
+- [bf qnamaker:build][6]
+- [bf luis:cross-train][7]
+- [bf luis:build][8]
+
+[1]:https://docs.microsoft.com/en-us/azure/bot-service/bot-builder-concept-adaptive-dialog-recognizers?view=azure-bot-service-4.0#cross-trained-recognizer-set
+[2]:https://aka.ms/adaptive-dialogs
+[3]:https://aka.ms/composer
+[4]:https://luis.ai
+[5]:https://aka.ms/qna-file-format
+[6]:./using-qnamaker-build.md
+[7]:../../luis/docs/using-cross-train-command.md
+[8]:../../luis/using-luis-build.md
+[10]:../../luis/docs/using-cross-train-command.md