readme: Add recommendation for local problem-spec repo (#1031)
readme: Add recommendation for local problem-spec repo (#1031)

The large number of exercises in now causing throttling to kick in
when using the generator tool. This can be avoided by using a local
copy of the `problem-specification` repository. I added an emphasis on
this point.

Resolves #1025
exercism/xgo,leenipper/xgo,exercism/xgo,leenipper/xgo