Add instructions for git-subtree, and an update script.

Author: SimonSapin

README.rst

@@ -1,3 +1,16 @@
+Importing into another repository with git-subtree
+==================================================
+
+To import the first time to a ``./css-parsing-tests`` sub-directory,
+run this from the top-level of a git repository::
+
+    git subtree add -P ./css-parsing-tests https://github.com/SimonSapin/css-parsing-tests.git master
+
+Later, to merge changes made in the upstream repository, run::
+
+    ./css-parsing-tests/update-subtree
+
+
 Result representation
 =====================
 

update-subtree

@@ -0,0 +1,2 @@
+#!/bin/sh
+git subtree pull -P $(dirname $0) https://github.com/SimonSapin/css-parsing-tests.git master