Sorry for my complete ignorance on this matter. I know what http://referencesource.microsoft.com/ aka http://sourceof.net is and I use it from time to time.
But I can't understand what this github repository at https://github.com/Microsoft/referencesource is for. I read its ReadMe.md file but I didn't understand anything.
Could you please explain what that repository holds and why would it be useful?
As the commentators mentioned in the comments to this question, the github repository at https://github.com/Microsoft/referencesource contains a sub-set of the .NET framework source code.
You can pull or fork from this repository.
I was misled to believe that it isn't the source code of the .NET framework because before posting this question, I peeked into several folders and source code but couldn't find the classes I was expecting. It turns out, I was looking at the wrong places. For e.g. I started looking for the System.Net.WebClient class, which is in the System.dll assembly in the System.Net folder but couldn't find it. Turns out, the class is present in the folder named System.