Add PR template and contribution guidelines

CONTRIBUTING.md

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+# How to contribute
+
+## Resources in the documentation
+
+* [Development quickstart](http://asciimoo.github.io/searx/dev/contribution_guide.html)
+* [Contribution guide](http://asciimoo.github.io/searx/dev/contribution_guide.html)
+
+## Submitting PRs
+
+Please follow the provided PR template when writing a description for your changes.
+
+Do not take criticism personally. When you get feedback, it is about your work,
+not your character, personality, etc. Keep in mind we all want to make the project better.
+
+When something is not clear, please ask questions to clear things up.
+
+If you would like to introduce a big architectural changes or do a refactoring
+either in the codebase or the development tools, please open an issue with a proposal
+first. This way we can think together about the problem and probably come up
+with a better solution.
+
+## Coding conventions and guidelines
+
+### Commit messages
+
+* Always write descriptive commit messages ("fix bug" is not acceptable).
+* Use the present tense ("Add feature" not "Added feature").
+* Use the imperative mood ("Move cursor to..." not "Moves cursor to...").
+* Limit the first line to 72 characters or less.
+* Include the number of the issue you are fixing.
+
+### Coding guidelines
+
+As a Python project, we must follow [PEP 8](https://www.python.org/dev/peps/pep-0008/) and [PEP 20](https://www.python.org/dev/peps/pep-0020/) guidelines.
+
+Furthermore, follow the Clean code conventions. The most important
+in this project are the following rules:
+
+* Simpler is better. [KISS principle](https://en.wikipedia.org/wiki/KISS_principle)
+* Be consistent.
+* Every function must do one thing.
+* Use descriptive names for functions and variables.
+* Always look for the root cause.
+* Keep configurable data high level.
+* Avoid negative conditionals.
+* Prefer fewer arguments.
+* Do not add obvious comment to code.
+* Do not comment out code, just delete lines.
+

PULL_REQUEST_TEMPLATE.md

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+## What does this PR do?
+
+<!-- MANDATORY -->
+
+<!-- explain the changes in your PR, algorithms, design, architecture -->
+
+## Why is this change important?
+
+<!-- MANDATORY -->
+
+<!-- explain the motivation behind your PR -->
+
+## How to test this PR locally?
+
+<!-- commands to run the tests or instructions to test the changes-->
+
+## Author's checklist
+
+<!-- additional notes for reviewiers -->
+
+## Related issues
+
+<!--
+Closes #234
+-->