Mention presubmit install hook in style guide

Adds a tip to the top of the style guide documentation that explains
how to install Pigweed's presubmit hook.

Change-Id: Idd50c612991efc83a55216464d3488f171e80783
diff --git a/docs/style_guide.rst b/docs/style_guide.rst
index fc0cad4..285ded8 100644
--- a/docs/style_guide.rst
+++ b/docs/style_guide.rst
@@ -8,6 +8,12 @@
 Style Guide and Conventions
 ===========================
 
+.. tip::
+  Pigweed runs ``pw format`` as part of ``pw presubmit`` to perform some code
+  formatting checks. To speed up the review process, consider adding ``pw
+  presubmit`` as a git push hook using the following command:
+  ``pw presubmit --install``
+
 ---------
 C++ style
 ---------