Document how to contribute to the project

[mrceresa]

In order to contribute to the project, several steps are needed:
(1) Install a compatible Unity version (2018.3.14f1)
(2) Download the project from github
(3) Import in unity the project
(4) Be sure that no assets are missing or need to be reimported
We should make a small wiki page with a step-by-step guide

[ryvix]

For 3 could you explain how exactly do we go about importing it into Unity?

Should we copy only the files in the Assets folder? What I tried was to create a new project, close it, copy all the files in this repo over it and then reopen it but got 599 errors which I'll make another issue about soon when I get a chance.

Some of the errors appeared to be related to 4 and once I reimported Mirror the number of errors were cut in half. It seemed like the standard assets might have to be reimported as well but there were still a number of other errors with trees and something about UnityShim in GitHub.

[mrceresa]

@ryvix I tried and I also get those errors if I download the project as a zip and import in unity. Investigating a bit, it seems to be related to some problems with the meta files and guid:
[1] https://blog.doppioslash.com/mildly-painful-solution-to-unity3ds-meta-guid-hell/
[2] https://thoughtbot.com/blog/how-to-git-with-unity
[3] https://forum.unity.com/threads/issues-downloading-cloning-project-from-github.322419/

I have, however, no idea on how to fix that because I've followed all the steps in [2]. I will investigate further in the following days