chore: add link from Lean 4 manual to FP in Lean

doc/SUMMARY.md

@@ -5,6 +5,7 @@
 - [Setting Up Lean](./quickstart.md)
   - [Extended Setup Notes](./setup.md)
 - [Theorem Proving in Lean](./tpil.md)
+- [Functional Programming in Lean](fplean.md)
 - [Examples](./examples.md)
   - [Palindromes](examples/palindromes.lean.md)
   - [Binary Search Trees](examples/bintree.lean.md)

doc/fplean.md

@@ -0,0 +1,7 @@
+Functional Programming in Lean
+=======================
+
+The goal of [this book](https://leanprover.github.io/functional_programming_in_lean/) is to be an accessible introduction to using Lean 4 as a programming language.
+It should be useful both to people who want to use Lean as a general-purpose programming language and to mathematicians who want to develop larger-scale proof automation but do not have a background in functional programming.
+It does not assume any background with functional programming, though it's probably not a good first book on programming in general.
+New content will be added once per month until it's done.