Add note for git bash and yarn bug

[Ragnoroct]

I've spent about 2-3 hours now trying to debug why yarn doesn't work for me when trying to run package.json script commands. I thought if anybody runs into the same issue this might help them.

It turns out it is yarnpkg/yarn#5717

I would constantly get /bin/bash: Files\git\bin\bash.exe: No such file or directory

After deleting the repo, uninstalling yarn and deleting all the cache and re-cloning it worked when using powershell. As soon as I accidentally used git bash though, even powershell would give the same error.

[Tyriar]

Since we're moving to Electron 6 soon we should also be upgrading our build to node 12 where this should be fixed. Let's not merge this in as we might forget to remove it then