Add instructions for Treesitter parsers

[gugahoi]

Not sure if this is needed or not but for a little while I thought I had a broken config. Turns out somehow I only had the typescript language parser installed and neotest-jest did not work repeatedly telling be there were no available tests. This might be helpful to explicitly call out for others in future.

[primeapple]

Makes sense, @haydenmeade , can we merge this?