docs: adds contributing file

Adds a contributing file with some guidelines for developing the project in order to contribute with Peru.

CONTRIBUTING.md

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+# Contributing
+
+We always like contributions here in ``peru``! 
+
+First of all, if you're looking for something to work or you have some idea for a new feature or you found a bug, then check out our [issue tracker](https://github.com/buildinspace/peru/issues) for this.
+
+In the issue, discuss your idea and implementation.
+
+Then if you want to make a contribution to ``peru`` then please raise a
+[Pull Request](https://github.com/buildinspace/peru/pulls) on GitHub.
+
+To help speed up the review process please ensure the following:
+
+- The PR addresses an open issue.
+- The project passes linting with ``flake8 peru tests``.
+- All tests are passing locally with (includes linting): ``python test.py``.
+- If adding a new feature you also add documentation.
+
+## Developing
+
+The minimal Python version supported is 3.5. If you are developing in newer versions, be aware of functions not backwards compatible. The Github Workflow will make this check in the pull request.
+
+To check out a local copy of the project you can [fork the project on GitHub](https://github.com/buildinspace/peru/fork)
+and then clone it locally. If you are using https, then you should adapt to it.
+
+```bash
+   $ git clone [email protected]:yourusername/peru.git
+   $ cd peru
+```
+
+This project uses ``flake8`` for linting.  To configure your local environment please install these development dependencies.
+
+```bash
+   $ pip install requirements-dev.txt
+```
+
+## Testing
+
+You can check that things are working correctly by calling the tests.
+
+```bash
+    $ python test.py -v
+    test_safe_communicate (test_async.AsyncTest) ... ok
+    test_basic_export (test_cache.CacheTest) ... ok
+    .
+    .
+    .
+    test_assert_contents (test_test_shared.SharedTestCodeTest) ... ok
+    test_create_dir (test_test_shared.SharedTestCodeTest) ... ok
+    test_read_dir (test_test_shared.SharedTestCodeTest) ... ok
+----------------------------------------------------------------------
+Ran 152 tests in 45.11s
+
+OK (skipped=1)
+````
+These checks will be run automatically when you make a pull request.
+
+You should always have a skipped test, because this is a platform specific tests.
+
+If you are working on a new feature please add tests to ensure the feature works as expected. If you are working on a bug fix then please add a test to ensure there is no regression.
+
+Tests are stored in ``peru/tests`` and verify the current implementation to see how your test will fit in.
+
+## Making a Pull Request
+
+Once you have made your changes and are ready to make a Pull Request please ensure tests and linting pass locally before pushing to GitHub.
+
+When making your Pull Request please include a short description of the changes, but more importantly why they are important. 
+
+Perhaps by writing a before and after paragraph with user examples.
+
+```
+# New feature short description here
+
+Closes #56
+
+**Changes**
+
+This PR includes a new feature that ...
+
+**Before**
+
+If a user tried to pull a repository ... 
+
+    ```python
+        > code example
+        >
+    ```
+**After**
+
+If a user tries to pull a repository now ...
+
+    ```python
+        > code example
+        >
+    ```
+```
+
+After that you should wait the review and perform possible changes in the submitted code.