Release Tutorial

[Vigir2]

This small PR adds the file CONTRIBUTING.md. This file contains a complete step-by-step tutorial on how to create and publish a new release of the package.

[OliRom]

Very clear guide! I approve this PR, but before merging it, I’d like to wait until I release the next version of QDecomp to follow this guide and see if anything is missing.

[OliRom]

Those tasks are handled by the make file.

Suggested change

	### 5. Tests and formatting
	Before creating the release, make sure that all **tests** are passing by running
	```bash
	pytest
	```
	in the root directory. Also make sure the repository has proper **formatting** by running
	```bash
	isort .
	black -l 100 .
	```
	from the root directory.
	### 5. Tests and formatting
	Before creating the release, make sure that all **tests** are passing by running
	```bash
	make test_cov

in the root directory. Also make sure the repository has proper formatting by running

make format

from the root directory.

[OliRom]

This code suggesting didn't render well because of the code blocks inside the commit suggestion (```). What I wanted to say is that the bash commands can be replaced with make test_cov to test with coverage and `make format` to run `isort` and `black`.

[Vigir2]

Maybe we should present both methods and not remove the individual commands, since they are more explicit than the make files about our formatting practices. What do you think?