Update README.md

Author: mrkm4ntr

README.md

@@ -5,3 +5,10 @@ This plugin provides network graph for [GitBucket](https://github.com/gitbucket/
 ## Installation
 
 Download jar file from [the release page](https://github.com/mrkm4ntr/gitbucket-network-plugin/releases) and put into `GITBUCKET_HOME/plugins`.
+
+## Build from source
+
+1. Install node.js and sbt
+1. `$ npm install`
+1. `$ npm run build`
+1. `$ sbt package`