Add pitfalls to Writing section

[rapodaca]

A collection of tricky cases and non-obvious conclusions about writing Dialect.

• bond split by cut
  • the bond itself
  • double-encoding is NOT required
    • can lead to bond compatibility errors that MUST be reported by reader
  • leave it out
    • doesn't matter which side
  • configuration
  • bond index, not atom index, sets configuration
  • examples