Document dependencies for running the build

[FagnerMartinsBrack]

The build requires npm, grunt and bower to be installed globally:

npm install grunt -g
npm install bower -g

[tholu]

@FagnerMartinsBrack No, it doesn't after merging #18 - grunt was an unneeded dependency I removed. The necessary dependencies (node, npm, bower) should also be installed via the frontend-maven-plugin plugin.

[tholu]

@FagnerMartinsBrack This can be closed now, everything is documented in the Contributing Guidelines.

[FagnerMartinsBrack]

Thanks @tholu