Adds development build instructions

[erikyo]

this PR will adds the basic build instructions in order to get the development environment working.

[bacoords]

I agree with all of this. It makes a lot of sense to keep this in the main readme and continue updating it as the plugin evolves.

[bacoords]

Makes sense.

[erikyo]

hey @Sephsekla, just came to my mind we can add something like
"postinstall": "npm run build"
to package.json in order to trigger automatically the build command after npm i was done, In this way the first build process could be even simpler! What do you think?

[bacoords]

@erikyo @Sephsekla Do we need additional work on this branch?

[erikyo]

@bacoords, I think it's completed, those are just the instructions to build the dev environment nothing more!