Contributing instructions and release workflow

[csala]

• Update Makefile with new release commands
• Reorganize Makefile by contepts
• Add detailed Contributing instructions

[csala]

@micahjsmith I had already pulled your changes, but the version that exists currently in master has all the different concepts mixed and it's difficult to find what uses what.

The new version is sorted as follows:

• All the targets that relate to the same thing are grouped together by concepts: clean, install, lint, test, docs and release.
• Things that are used by others go first, so the clean group is the first one, and then inside each group the most basic things go first, and the complex ones are last.

The ordering of the groups after clean can be discussed, as there no dependencies between them.