docs: add development.md with a section on how to add a fork as a remote (#11)

docs/development.md

@@ -0,0 +1,29 @@
+# Developing hugo-theme-readable
+
+## Adding your fork as an additional Git remote to a hugo-theme-readable Git submodule
+
+The easiest way to test your changes is with an existing Hugo website that already uses this theme. You can use your own or checkout the [starter template](https://github.com/cjtheham/hugo-starter-readable).
+
+The following steps show you how to add your fork as an additional Git remote to the checked out Git submodule.
+
+This way you can work with the hugo-theme-readable code directly, rather than experimenting in your own 'layouts' directory and copying code pieces around.
+
+```bash
+# If you haven't done so: init and update the submodule. This will populate themes/readable.
+git submodule init
+git submodule update
+
+cd themes/readable/
+
+# Add your fork as the remote 'fork'.
+git remote add fork git@github.com:<your-username>/hugo-theme-readable.git
+
+# Create a new branch.
+git checkout -b my-change
+
+# Make your changes directly to files in themes/readable.
+
+# Anytime you want to push, replace the default 'origin' with 'fork', e.g.
+# push your work on 'my-change' as a new branch to your fork.
+git push -u fork my-change
+```