Added CONTRIBUTING, ISSUE and PULL_REQUEST TEMPLATE

+# Contributing to SDWebImage
+
+We want to make contributing to this project as easy and transparent as possible. Here are a few guidelines for making all our lives easier.
+
+## Asking questions
+
+We don't use GitHub as a support forum. For any usage questions that are not specific to the project itself, please ask on [Stack Overflow](https://stackoverflow.com/) instead. By doing so, you'll be more likely to quickly solve your problem, and you'll allow anyone else with the same question to find the answer. This also allows maintainers to focus on improving the project for others.
+
+## Reporting Issues
+
+A great way to contribute to the project is to send a detailed issue when you encounter an problem.
+It is very important to check for the same problem or suggestion in the project's issue list first. If you find a match, just add a small comment there. 
+Doing this helps prioritize the most common problems and requests.
+
+When reporting issues, please include the following:
+
+- The platform name and version (e.g. iOS 8.1)
+- The library version
+- The integration method (e.g. CocoaPods/Carthage/manually)
+- The version of Xcode you're using
+- The full output of any stack trace or compiler error
+- A small demo project that replicates the issue (especially if the way to reproduce the issue is not straight-forward)
+- Any other details that would be useful in understanding the problem
+
+This information will help us review and fix your issue faster.
+
+
+Please do not be offended if we close your issue and reference this document. 
+If you believe the issue is truely a fault in the project’s codebase, re-open it.
+
+## Pull Requests
+
+We gladly accept any PR's assuming they are well written, documented ( if necessary ) and preferably have test code. 
+If you're unsure if we'll accept a new feature please open an issue requesting it and we can have a discussion before you code and submit a PR.
+
+Checklist:
+- Fork the repo and create your branch from the latest master (to minimize the conflicts)
+- If you've added code that should be tested, add tests.
+- If you've changed APIs, update the documentation.
+- Ensure the test suite passes.
+- Make sure your code lints (pod lib lint)
+

+### New Issue Checklist
+
+* [ ] I have read and understood the [CONTRIBUTING guide](https://github.com/rs/SDWebImage/blob/master/.github/CONTRIBUTING.md)
+* [ ] I have read the [Documentation](http://cocoadocs.org/docsets/SDWebImage/)
+* [ ] I have searched for a similar issue in the [project](https://github.com/rs/SDWebImage/issues) and found none
+
+### Issue Info
+
+ Info                    | Value                               |
+-------------------------|-------------------------------------|
+ Platform Name           | e.g. ios / tvos
+ Platform Version        | e.g. 8.0
+ SDWebImage Version      | e.g. 3.7.6
+ Integration Method      | e.g. carthage / cocoapods / manually
+ Xcode Version           | e.g. Xcode 7.3
+ Repro rate              | e.g. all the time (100%) / sometimes x% / only once
+ Repro with our demo prj | e.g. does it happen with our demo project?
+ Demo project link       | e.g. link to a demo project that highlights the issue
+
+### Issue Description and Steps
+
+Please fill in the detailed description of the issue (full output of any stack trace, compiler error, ...) and the steps to reproduce the issue.
+

+### New Pull Request Checklist
+
+* [ ] I have read and understood the [CONTRIBUTING guide](https://github.com/rs/SDWebImage/blob/master/.github/CONTRIBUTING.md)
+* [ ] I have read the [Documentation](http://cocoadocs.org/docsets/SDWebImage/)
+* [ ] I have searched for a similar pull request in the [project](https://github.com/rs/SDWebImage/pulls) and found none
+
+* [ ] I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)
+* [ ] I have added the required tests to prove the fix/feature I am adding
+* [ ] I have updated the documentation (if necesarry)
+* [ ] I have run the tests and they pass
+* [ ] I have run the lint and it passes (`pod lib lint`)
+
+This merge request fixes / reffers to the following issues: ...
+
+### Pull Request Description
+
+...
+