Update docs to mention GERRIT_SITE env variable

Change-Id: I869842e4dd8fbab5462e671bb270e1cbacea8e07

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index c5698be..cd130de 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -4,6 +4,9 @@
 Environment
 -----------
 
+When a hook is run by Gerrit, the environment will have `GERRIT_SITE` set to
+the full path of the root of the Gerrit site ($site_path).
+
 When a project is given to a hook, the environment will have `GIT_DIR` set to
 the full path of the affected git repository so that git commands can be easily
 run.