Readme updates: rule compatibilties + fs2 sync compiler instructions

[samspills]

• adds a little table showing the current scala 3 compatibility for all the rules.
  • I'm not sure if the rules could easily be updated to support scala 3. This is just what I found in my experiments at $work
• documents the synthetics flag necessary (??) for the fs2 sync compiler rule
  • I had to enable synthetics to get this rule to work in my experiments, and I figured that out via this comment: Create FS2 module, add SyncCompiler rule #22 (comment)

[armanbilge]

thank you Sam 🙏