update the README's description of make_test_manifest usage
update the README's description of make_test_manifest usage

Note that this change only re-wraps lines with wording changes.  This
leaves the paragraph wrapping in an odd state.  A later commit will
re-line-wrap the whole document to clean it up.

rhansen/rpstir,rhansen/rpstir,rhansen/rpstir,rhansen/rpstir,rhansen/rpstir