Developer documentation

Add DEVELOPMENT.md, which contains a rough development workflow (what I did to run it).

*We should definitely add Yarn - raw NPM behaved differently on two different machines due to slightly different configuration. Yarn would prevent this and would also allow us to more cleanly manage packages that must be installed with special flags like sqlite3. Otherwise I expect a few devs will be frustrated with these instructions as they don't anticipate for this kind of thing.