Add local development instructions

I had a couple of issues getting the project to run, thought this could
be useful to others.

Author: jstasiak

Diff for: README.md

@@ -1,2 +1,18 @@
 # pythonlodz.org
 Repo for keeping the source code of our official website
+
+## How to run the website locally
+
+1. [Install Hugo](https://gohugo.io/installation/).
+2. Clone this repository.
+3. On new clone and when there are submodule updates:
+
+   ```
+   git submodule update --init --depth 1
+   ```
+4. Run the Hugo development server:
+
+   ```
+   hugo server
+   ```
+5. Open http://127.0.0.1:1313.