Create contribution guideline

CONTRIBUTING.md

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+Feel like our bot is missing a feature? We welcome your pull requests! Few pointers for contributions:
+
+- Create your PR against the `develop` branch, not `master`.
+- New features need to contain unit tests and must be PEP8 conformant (max-line-length = 100).
+
+If you are unsure, discuss the feature on our [Slack](https://join.slack.com/t/highfrequencybot/shared_invite/enQtMjQ5NTM0OTYzMzY3LWMxYzE3M2MxNDdjMGM3ZTYwNzFjMGIwZGRjNTc3ZGU3MGE3NzdmZGMwNmU3NDM5ZTNmM2Y3NjRiNzk4NmM4OGE)
+or in a [issue](https://github.com/gcarq/freqtrade/issues) before a PR.
+
+Before sending the PR:
+
+## Run unit tests
+
+All unit tests must pass. If a unit test is broken, change your code to make it pass. It means you have introduced a regression
+
+**Test the whole project**
+```bash
+pytest freqtrade
+```
+
+**Test only one file**
+```bash
+pytest freqtrade/tests/test_<file_name>.py
+```
+
+**Test only one method from one file**
+```bash
+pytest freqtrade/tests/test_<file_name>.py:test_<method_name>
+```
+## Test if your code is PEP8 compliant
+**Install packages** (If not already installed)
+```bash
+pip3.6 install flake8 coveralls
+``` 
+**Run Flake8**
+```bash
+flake8 freqtrade
+```
+
+