Document monorepo migration

[gabor-meszaros]

The Proton Mail Desktop app code was moved to WebClients. This hasn't been documented here that leads to confusion.

A contributing guideline is also added to help our users to know how to reach us.

[cuthix]

Not applicable here IMHO.. If we want to moderate we should specify in monorepo

Suggested change

	## Code contribution
	We are grateful if you can contribute directly with code. In that case there is nothing else to do than to open a pull request.
	The following is worthwhile noting
	- The project is primarily developed on an internal repository, and the one on GitHub is only a mirror of it. For that reason, the merge request will not be merged on GitHub but added to the project internally. We are keeping the original author in the change set to respect the contribution.
	- The application is used on numerous platforms. To have higher chance your change to be accepted, consider all supported dependencies.
	- Give detailed description of the issue, preferably with test steps to reproduce the original issue, and to verify the fix. It is even better if you also extend the automated tests.
	### Contribution policy
	By making a contribution to this project:
	1. You assign any and all copyright related to the contribution to Proton AG;
	2. You certify that the contribution was created in whole by you;
	3. You understand and agree that this project and the contribution are public and that a record of the contribution (including all personal information you submit with it) is maintained indefinitely and may be redistributed with this project or the open source license(s) involved.