README.md needs updating

[Finchiedev]

The following changes need to be made to README.md:

• Add install instructions from Bintray, GitHub releases, Software Centre (CCGS only)
• Decrease length of installation instructions, they don't need to be that long
• Update developing guide with full process from download to development environment
• Update feature list to reflect current status

[Finchiedev]

For the development section:

• How to create a release using release-it
• How to start Electron
• How to build for a specific target
• How to use yarn

[Finchiedev]

We should also link:

• All the new GitBook documentation
• The Riot/Gitter chat for non-CCGS discussion & help