Document PRE_COMMIT environment variable

Document this variable, exposed since `v2.5.0`[1], as a way to determine
if `pre-commit` is running. Because It took a bit of searching in the
`pre-commit` repo to discover the existence of this variable.

Link: pre-commit/pre-commit@2f25085 [1]

Author: matthewhughes934

sections/advanced.md

@@ -885,3 +885,8 @@ tag or SHA) and will cache based on that.  Using a branch name (or `HEAD`) for
 the value of [`rev`](#repos-rev) is not supported and will only represent the state of
 that mutable ref at the time of hook installation (and will *NOT* update
 automatically).
+
+## Detecting when `pre-commit` is running
+
+You can check for the existence of the `PRE_COMMIT` environment variable within
+a hook to determine if it's being run from `pre-commit`.