Add build instructions

README.md

@@ -2,3 +2,18 @@ This is the source for https://electioncal.us which provides election
 timeline information and makes it available through the website and
 icalendar files.
 
+# Building the site
+
+The site is statically hosted by GitHub. To test it locally, first install the
+python requirements with:
+
+    pip install -r requirements.txt
+
+Once those are installed, build the site with:
+
+    python scripts/build_site.py
+
+This generates all of files served statically under the `site` folder. To serve
+them locally do:
+
+    python -m http.server --directory site/ 8080