docs(*): Add issue, pr templates; Add contributor guidelines

Author: benmarten

CONTRIBUTING.md

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+# How to Contribute
+
+Here are some important resources:
+
+  * [Leanplum Docs](http://leanplum.com/docs) The official Leanplum docs.
+  * Support: Get help at [email protected].
+
+## Submitting changes
+
+Please consider following guidelines before creating a PR:
+
+- Confirm the intention and design of your change by reaching out to your Customer Success Manager or to Leanplum Dev Team through a new Github Issue.
+- Start creating your pull request from `develop` branch
+- Please send a GitHub Pull Request with a clear list of what you've done (read more about [pull requests](https://help.github.com/articles/about-pull-requests/)).  
+- Add unit tests for the code that you have touched.
+- Please follow the existing coding conventions (which are mostly standard of the default IDE).
+- Make sure all of your commits are atomic (one feature per commit).
+- Please adhere to the conventional changelog commit style: https://github.com/commitizen/cz-cli That allows us to easily create change logs.

ISSUE_TEMPLATE.md

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+## Expected Behavior
+
+
+## Actual Behavior
+
+
+## Steps to Reproduce the Problem
+
+  1.
+  1.
+  1.
+
+## Specifications
+
+  - Version:
+  - Platform:
+  - Subsystem:
+

PULL_REQUEST_TEMPLATE.md

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+Fixes #
+
+## Proposed Changes
+
+  -
+  -
+  -
+