Update README with changes made in #7

Author: dentarg

README.md

@@ -1,6 +1,8 @@
 # Gem Compare
 
-For pull requests with a title matching `/Bump (.+) from (.+) to (.+)/`, it posts a comment with the output from `gem compare` and `gem compare --diff`. Uses https://rubygems.org/gems/gem-compare (https://github.com/fedora-ruby/gem-compare).
+Reads the lockfile (`Gemfile.lock` by default) diff and posts comments with the output from `gem compare` and `gem compare --diff` for the gems that changed version.
+
+Uses https://rubygems.org/gems/gem-compare (https://github.com/fedora-ruby/gem-compare).
 
 To use in your repo, add a workflow like this: