Add instructions for building webpage locally (#80)

README.MD

@@ -1,3 +1,23 @@
 # NorthstarTF
 
 The NorthstarTF repository holds the code for the [northstar.tf](https://northstar.tf) website.
+
+## Developing
+
+To render the webpage locally you will need to have [`yarn`](https://yarnpkg.com/) installed.
+
+Then run
+
+```sh
+yarn
+```
+
+once to install the necessary dependencies.
+
+After that
+
+```sh
+yarn dev
+```
+to build the webpage and run a local webserver displaying the changes.
+It should live update everytime you perform any changes display them.