Add clarifying information to "Hacking on Atom Core"

[boubascript]

Summary

There are a couple of issues with the Hacking the Atom Core page.

The section makes several assumptions that should be either stated or fixed:

• Users have already installed Atom.
• Users understand the significance of having a local copy of Atom source code and know that Atom will reflect code changes in development mode.
• Users understand the purpose of bootstrap and build scripts.

Additional problems:

• The atom --dev path-to-open instruction and its following note suggest that Atom can only now be run in dev mode and path-to-open is not explained at all.
• Dependencies required for the bootstrap script dependencies are not listed in the bootstrap section but are instead included below, in the build section.

Motivation

Adding clarifying information to "Hacking on Atom Core" will ease the process for new contributors to Atom in the future. Right now, the documentation is unclear and very easy to misread.

Describe alternatives you've considered

In this case there are no alternatives: the Flight Manual and, in particular, this section, should be the single source of truth for new contributors since it is referenced in the Atom contributing document.

Additional context

Related to #482.

#438 partially addresses the problems with the ATOM_DEV_RESOURCE_PATH documentation, but the section can still be made clearer.

The Manual also suggests any Node version above 6 should be fine, but only Node version 10 successfully ran the bootstrap script (both earlier and later versions didn't work).