Diagram(s) on the amendment process

[mDuo13]

A flowchart or something to represent the amendment process might aid understanding. One or more diagrams could try to encompass:

• the "happy path" from an amendment being open for voting, to gaining majority, to becoming enabled
• the "rocky path" path where an amendment gains/loses majority several times before finally getting enabled
• the "unhappy path" where an amendment eventually gets removed from the source without ever being enabled
• how old amendments can be "unconditionalized" which removes support for legacy ledger playback
• how old servers from before the amendment become amendment blocked if an amendment they don't support gets enabled

[mDuo13]

There's a diagram in the legacy Feature adoption Ripple wiki page that is not very good but might serve as inspiration for part of this (note, that page was speculative and things don't actually work as described there)

[mDuo13]

#1667 should fix this issue

[oeggert]

Will work on an updated diagram after #1667 process is finalized.