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/new-hooks.md

@@ -93,6 +93,9 @@ For example:
     types: [text]
 ```
 
+To determine if an application was spawned as a `pre-commit` hook you can check
+for the existence of the `PRE_COMMIT` environment variable.
+
 ## Developing hooks interactively
 
 Since the [`repo`](#repos-repo) property of `.pre-commit-config.yaml` can refer to anything