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

@@ -4,6 +4,9 @@ installable package (gem, npm, pypi, etc.) or exposes an executable, it can be
 used with pre-commit. Each git repo can support as many languages/hooks as you
 want.
 
+_new in 2.5.0_: `pre-commit` sets the `PRE_COMMIT=1` environment variable
+during hook execution.
+
 The hook must exit nonzero on failure or modify files.
 
 A git repo containing pre-commit plugins must contain a `.pre-commit-hooks.yaml`