Add DEVELOPING.md and Vagrantfile to improve developing documentation and flow.

[phoeinx]

This infrastructure PR contains:

• 1. A new documentation file: DEVELOPING.md, which serves to document helpful knowledge, commands and setup instructions for better developing flow. (During my GSoC project I've started documenting helpful guidance I've received from @drrtuy and things that I found out, which helped in my understanding and development and wanted to share.)
• 2. A Vagrantfile which serves to set up a local VM to develop in for developers on non-Linux machines. Currently, the Vagrantfile is configured for ARM-based Macs, but could probably be easily adapted for other systems. More documentation can be found in the Development Setup with Local Linux VM for ARM-based Mac part of the DEVELOPING.md.