DOC CONTRIBUTING.md adding coding guidelines (#97)

Author: leszekhanusz

CONTRIBUTING.md

@@ -57,16 +57,44 @@ And you ready to start development!
 
 <!-- TODO: Provide environment.yml file for conda env -->
 
+## Coding guidelines
+
+Several tools are used to ensure a coherent coding style.
+You need to make sure that your code satisfy those requirements
+or the automated tests will fail.
+
+- [black code formatter](https://github.com/psf/black)
+- [flake8 style enforcement](https://flake8.pycqa.org/en/latest/index.html)
+- [mypy static type checker](http://mypy-lang.org/)
+- [isort to sort imports alphabetically](https://isort.readthedocs.io/en/stable/)
+
+On Linux or MacOS, you can fix and check your code style by running
+the Makefile command `make check` (this is also checked by running
+the automated tests with tox but it is much faster with make)
+
+In addition to the above checks, it is asked that:
+
+- [type hints are used](https://docs.python.org/3/library/typing.html)
+- tests are added to ensure complete code coverage
+
 ## Running tests
 
 After developing, the full test suite can be evaluated by running:
 
 ```sh
-pytest tests --cov=gql -vv
+pytest tests --cov=gql --cov-report=term-missing -vv
 ```
 
-If you are using Linux or MacOS, you can make use of Makefile command
-`make tests`, which is a shortcut for the above python command.
+Please note that some tests which require external online resources are not
+done in the automated tests. You can run those tests by running:
+
+```sh
+pytest tests --cov=gql --cov-report=term-missing --run-online -vv
+```
+
+If you are using Linux or MacOS, you can make use of Makefile commands
+`make tests` and `make all_tests`, which are shortcuts for the above
+python commands.
 
 You can also test on several python environments by using tox.
 
@@ -91,7 +119,24 @@ conda install -c conda-forge tox-conda
 
 This install tox underneath so no need to install it before.
 
-Then uncomment the `requires = tox-conda` line on `tox.ini` file.
+Then add the line `requires = tox-conda` in the `tox.ini` file under `[tox]`.
 
 Run `tox` and you will see all the environments being created
 and all passing tests. :rocket:
+
+## How to create a good Pull Request
+
+1. Make a fork of the master branch on github
+2. Clone your forked repo on your computer
+3. Create a feature branch `git checkout -b feature_my_awesome_feature`
+4. Modify the code
+5. Verify that the [Coding guidelines](#coding-guidelines) are respected
+6. Verify that the [automated tests](#running-tests) are passing
+7. Make a commit and push it to your fork
+8. From github, create the pull request. Automated tests from travis
+and coveralls will then automatically run the tests and check the code coverage
+9. If other modifications are needed, you are free to create more commits and
+push them on your branch. They'll get added to the PR automatically.
+
+Once the Pull Request is accepted and merged, you can safely
+delete the branch (and the forked repo if no more development is needed).