cmd/go: add README and access log to cache directory
The README is there to help people who stumble across the directory.
The access log is there to help us evaluate potential algorithms for
managing and pruning cache directories. For now the management
is manual: users have to run "go clean -cache" if they want the cache
to get smaller.
As a low-resolution version of the access log, we also update the
mtime on each cache file as they are used by the go command.
A simple refinement of go clean -cache would be to delete
(perhaps automatically) cache files that have not been used in more
than one day, or some suitable time period.