Document the pre-commit hook modes.

Signed-off-by: format 2020.01.12 <github.com/ChrisCummins/format>

[Exported from 4312fc435ec599e42fcd4ef8e5286772ab029d79]

tools/format/README.md

@@ -13,10 +13,14 @@ Features:
 
   * Automated code styling of C/C++, Python, Java, SQL, JavaScript, HTML,
     CSS, go, and JSON files.
-  * Support for `.formatignore` files to mark files to be excluded from 
-    formatting. The syntax of ignore files is similar to `.gitignore`, e.g. a 
-    list of patterns to match, including (recursive) glob expansion, and 
-    patterns beginning with `!` are un-ignored. 
+  * Support for `.formatignore` files to mark files to be excluded from
+    formatting. The syntax of ignore files is similar to `.gitignore`, e.g. a
+    list of patterns to match, including (recursive) glob expansion, and
+    patterns beginning with `!` are un-ignored.
+  * A `--pre_commit` mode which formats files that have been staged for commit,
+    and stages changes. The commit is rejected if a partially-staged file is
+    modified. Enforce the use of pre-commit mode (and add a "Signed off" footer
+    to commits) by running `--install_pre_commit_hook`.
   * Persistent caching of "last modified" timestamps for files to minimize the
     amount of work done.
   * A process lock which prevents races when multiple formatters are launched

tools/format/format.py

@@ -27,6 +27,10 @@
     formatting. The syntax of ignore files is similar to `.gitignore`, e.g. a
     list of patterns to match, including (recursive) glob expansion, and
     patterns beginning with `!` are un-ignored.
+  * A `--pre_commit` mode which formats files that have been staged for commit,
+    and stages changes. The commit is rejected if a partially-staged file is
+    modified. Enforce the use of pre-commit mode (and add a "Signed off" footer
+    to commits) by running `--install_pre_commit_hook`.
   * Persistent caching of "last modified" timestamps for files to minimize the
     amount of work done.
   * A process lock which prevents races when multiple formatters are launched