Improve development setup documentation

[pR0Ps]

Added information regarding:

• Required php.ini settings
• Setting up the database
• Making the storage folder available from the application
• Using the 'array' cache driver (easier for dev than memcached)
• Where mail logs are stored and why they're needed

Slight whitespace changes to the .env.example file were made to allow bash to source it, enabling an easier database setup.