doc(hack): Add explanation to hack and test HLS

[sir4ur0n]

I think this kind of documentation, even though it may seem obvious to HLS maintainers, are helpful to newcomers who wish to give it a shot hacking HLS 😄

[jneira]

This was not my workflow but it is a good one
I am installing hls globally in my default bin path or, if i am hacking on a pr i change the hls path in the vscode config to a fixed external location and install with cabal --installdir=/path/to/custom/hls install -w ghc-xxx. This way i have them stored in case i have to switch between branches or compare some behaviour between them.
cabal install does not reuse the entire cache with cabal build so sometimes you have to wait though.