Update README.md (Azure#510)

* Update README.md

Add references to `azdev extension update-index` command.

* Code review feedback.

* Update README.md

README.md

@@ -37,6 +37,10 @@ Add your extension to the index to make it available in these CLI commands:
 
 ### How to generate sha256digest for an index.json entry?
 
+If you use `azdev extension update-index` the command will calculate the SHA256 digest for you. For more information visit https://github.com/Azure/azure-cli-dev-tools.
+
+As a fallback:
+
 MacOS
 ```
 shasum -a 256 path_to_whl.whl
@@ -51,6 +55,10 @@ Note: Hash should be in lowercase in index.json otherwise CI will fail.
 
 ### How to fill in the metadata for an index.json entry?
 
+The `azdev extension update-index` command can be used to simplify the process of updating the index file. Run `azdev extension update-index <URL>` where URL is the fully-qualified URL to your published extension WHL. This will gather the appropriate metadata and add an entry for your extension to the index. For more information visit https://github.com/Azure/azure-cli-dev-tools.
+
+As a fallback:
+
 The metadata needed to be filled is a combination of the contents present in:
 - `metadata.json` located in your unzipped extension artifact (`.whl` file) in the `<package>-<version>.dist-info` directory. This metadata is garnered from the `setup.py` folder.
 - `azext_metadata.json` (if it exists) under your extension.