README: Add credit

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

README.rst

@@ -4,4 +4,7 @@ pyenv
 
 pyenv container image for use in `Patchwork`__ development and CI.
 
+Based on `vicamo/docker-pyenv`__.
+
 .. __: https://github.com/getpatchwork/patchwork
+.. __: https://github.com/vicamo/docker-pyenv