Refresh the Jumpy git README.md

[MaxCWhitehead]

Description

Our README is a bit stale, couple of notes for future reference when we want to refresh it:

• Should update Status header, not really being rewritten for Bevy anymore. Now we are lightly based on Bevy, but this is fully wrapped up / encapsulated by bones.
• Might be good to mention modding / lua plugin support, it isn't obvious that this is one of our focuses from github landing page.
• Remove reference to jumpy_core as separate crate, Jumpy is now a single crate.
• ??? probably other things

[erlend-sh]

No more ‘live on Kickstarter’ :D