Elaborate on custom bundle instructions

[birkskyum]

For custom builds, it seems easier to just clone the repo, aligned with the contrib guide, instead of the somewhat atypical approach to npm installing it and go to the node_modules/plotly.js for making custom builds

[archmoj]

How about adding both install and clone options with some basic information of the advantages of each one?
E.g. installing may be simpler for some users and they get latest official version instead an unstable/unreleased work on master.

[birkskyum]

@archmoj , good catch! I've updated the instructions to include a version (tags)

[birkskyum]

I made a note, to check the CONTRIBUTING.md list of prerequisites for node/npm version after the clone of a specific version, so that it'll be aligned.