Add example repo and encourage people to use it

We previously encouraged people to use a test resource repository.
However we did not clarify that people should *copy* said test
resource.  The examples modify this test resource and leave it in a
state where subsequent clar runs will fail.  This adds a new example
repository (a copy of testrepo.git) and points people to use it,
instead.

(This is in response to issue https://github.com/libgit2/libgit2/pull/1757.)