Update README.md

Author: rct225

README.md

@@ -3,3 +3,18 @@
 This repository is for the US CMS Software & Computing website.
 
 Changes can be made by performing a pull request from this repository.
+
+# Testing pages locally
+
+You can test your changes out using the Jekyll docker image:
+```bash
+docker run --rm -it \
+      --volume="$PWD:/srv/jekyll"  \
+      -p 4000:4000 \
+      jekyll/builder:stable jekyll serve --incremental
+```
+*Note* - you need to run the command from the top-level of your local repo.
+
+This will mount your checked out copy of this repo, then build and start the
+jekyll server mapping it to port 4000 on your computer. You can make changes
+locally and view them at http://localhost:4000