Document adding a new method to `IsExprBuilder`

[langston-barrett]

To add a new method to IsExprBuilder, one would want to:

• Add a corresponding constructor to What4.Expr.App
• Implement it for ExprBuilder
• Add support for serializing it to SMT
• Potentially add support for parsing it from SMT (see Use What4.Serialize.Parser to parse solver models/values. #228 and w4smt2: A bad SMT solver #343)
• Add some tests, perhaps in ExprsTest
• Add it to the CHANGELOG

We should document all this somewhere.