Reworking README file

[bugsysop]

I’m reworking the README file for my use.

• Presentation of Mangane on top
• References to Pleroma/Akkoma
• Wording for install instruction
• References to Soapbox
• Completed License en credits

Todo

• Reduce space between screenshots
• Update Admin FE screenshots
• Wording for activation section
• Verify and correct some external links

You can see the result here https://github.com/bugsysop/Mangane/blob/update-doc-1/README.md

Are you interested by a push to upstream?

[Dehelssey]

Seems quite OK to me. I was already planning on new screens when new home UI is deployed !