docs: add PR and issue templates

.github/issue_template.md

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+### Make sure to check the demo app(s) for sample usage 
+
+### Make sure to check the existing issues in this repository 
+
+### If the demo apps cannot help and there is no issue for your problem, tell us about it
+Please, ensure your title is less than 63 characters long and starts with a capital
+letter.
+
+### Which platform(s) does your issue occur on?
+- iOS/Android/Both 
+- iOS/Android versions 
+- emulator or device. What type of device?
+
+### Please, provide the following version numbers that your issue occurs with:
+
+- CLI: (run `tns --version` to fetch it)
+- Cross-platform modules: (check the 'version' attribute in the
+`node_modules/tns-core-modules/package.json` file in your project)
+- Runtime(s): (look for the `"tns-android"` and `"tns-ios"` properties in the `package.json` file of your project)
+- Plugin(s): (look for the version numbers in the `package.json` file of your
+project and paste your dependencies and devDependencies here)
+
+### Please, tell us how to recreate the issue in as much detail as possible. 
+Describe the steps to reproduce it.
+
+### Is there any code involved? 
+ - provide a code example to recreate the problem 
+ - (EVEN BETTER) provide a .zip with application or refer to a repository with application where the problem is reproducible.

.github/pull_request_template.md

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+<!--
+We, the rest of the NativeScript community, thank you for your
+contribution! 
+To help the rest of the community review your change, please follow the instructions in the template.
+-->
+
+<!-- PULL REQUEST TEMPLATE -->
+<!-- (Update "[ ]" to "[x]" to check a box) -->
+
+## PR Checklist
+
+- [ ] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
+- [ ] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
+- [ ] All existing tests are passing
+- [ ] Tests for the changes are included
+
+## What is the current behavior?
+<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->
+
+## What is the new behavior?
+<!-- Describe the changes. -->
+
+Fixes/Implements/Closes #[Issue Number].
+
+<!-- If this PR contains a breaking change, please describe the impact and migration path for existing applications below. -->
+
+<!-- 
+BREAKING CHANGES:
+
+
+[Describe the impact of the changes here.]
+
+Migration steps:
+[Provide a migration path for existing applications.]
+-->
+