README.md: add link target to status badge

In commit c735fc7 of PR #31 we
updated the CI status badge in our README.md file to reflect the
fact that we are now using GitHub Actions rather than Travis CI.

However, we didn't include a link target for the new badge, so
we do that now.

Author: chrisd8088

README.md

@@ -1,8 +1,9 @@
 # gitobj
 
-![CI status][1]
+[![CI status][ci_badge]][ci_url]
 
-[1]: https://github.com/git-lfs/gitobj/workflows/CI/badge.svg
+[ci_badge]: https://github.com/git-lfs/gitobj/workflows/CI/badge.svg
+[ci_url]: https://github.com/git-lfs/gitobj/actions?query=workflow%3ACI
 
 Package `gitobj` reads and writes loose and packed Git objects.