Document running standalone

Fixes #32

README.md

@@ -156,6 +156,16 @@ To copy your default hooks into existing repos:
 $ git init
 ```
 
+### How can I run this standalone, like part of a CI process?
+
+Fit Commit can be run outside of a Git hook context with a simple shell script:
+
+```sh
+$ GIT_BRANCH_NAME=branch_name COMMIT_MESSAGE_PATH=path/to/message rbenv exec ruby -rrubygems -e 'require "fit_commit"; FitCommit.run'
+```
+
+It exits with an error code if any errors are present, which will fail a build if it's part of a CI run. The configuration should be modified for CI to be more lenient, otherwise false positives will break the build too often.
+
 ### Who decided these rules?
 Fit Commit aims to enforce *community standards*. The two influential guides are: