update: SUMMARY and add type classes

Author: thelissimus

haskell-for-rustaceans/src/SUMMARY.md

@@ -11,4 +11,8 @@
   - [Data Types](./ch02-01-data-types.md)
   - [Control Flow](./ch02-02-control-flow.md)
   - [Functions]()
-- [Borrowing]()
+- [Linear Types]()
+- [Type classes]()
+  - [Orphan instances]()
+  - [Overlapping instances]()
+  - [Incoherent instances]()