[DOCS, SETUP] Adding some setup instructions

[geleeroyale]

While deploying my first sourcecred instance (for Giveth 🤗), I noticed the following things:

• Symlinks are not supported in Windows in the same way they are supported in Linux, that means yarn site will fail on Windows - the other commands work, so maybe add a disclaimer or additional tip for windows users to use WSL2 (Windows Subsystem for Linux).
  [Suggested Fix] To install sourcecred on a Windows machine, please use a Linux shell (i.e. WSL2)

• The github pages deployment seems to automatically use github pages branch when it should build from master branch.
  [Suggested Fix] Set the branch in Settings to deploy GitHub pages from the master branch

• Deployment seems to need a custom domain to work (standard URLs for GH pages append the repo name to the web address, causing a 404 when looking for index.html in the subdirectory that is named after the repo)
  [Suggested Fix] Set a custom URL in gh-pages settings

[EDIT] Finally, some more clarification around the actual deployment would be nice. I was confused initially by the fact that we don't need to set up a server for this, until I figured out that all server side events seem to be integrated purely via GitHub actions.