Adds documentation about function contracts

Signed-off-by: Felipe R. Monteiro <felisous@amazon.com>

Author: feliperodri

CODEOWNERS

@@ -46,6 +46,7 @@
 /src/goto-harness/ @martin-cs @chrisr-diffblue @peterschrammel
 /src/goto-instrument/ @martin-cs @chrisr-diffblue @peterschrammel
 /src/goto-instrument/code_contracts.* @tautschnig @feliperodri @SaswatPadhi
+doc/cprover-manual/contracts* @tautschnig @feliperodri @SaswatPadhi
 /src/goto-diff/ @tautschnig @peterschrammel
 /src/jsil/ @kroening @tautschnig
 /src/memory-analyzer/ @tautschnig @chrisr-diffblue