Add links to the docs.rs API documentation.

cranelift/docs/index.rst

@@ -12,6 +12,28 @@ Contents:
    regalloc
    compare-llvm
 
+Rust Crate Documentation
+========================
+
+`cretonne <https://docs.rs/cretonne/>`_
+    This is the core code generator crate. It takes Cretonne IR as input
+    and emits encoded machine instructions, along with symbolic relocations,
+    as output.
+
+`cretonne-wasm <https://docs.rs/cretonne-wasm/>`_
+    This crate translates WebAssembly code into Cretonne IR.
+
+`cretonne-frontend <https://docs.rs/cretonne-frontend/>`_
+    This crate provides utilities for translating code into Cretonne IR.
+
+`cretonne-native <https://docs.rs/cretonne-native/>`_
+    This crate performs auto-detection of the host, allowing Cretonne to
+    generate code optimized for the machine it's running on.
+
+`cretonne-reader <https://docs.rs/cretonne-reader/>`_
+    This crate translates from Cretonne IR's text format into Cretonne IR
+    in in-memory data structures.
+
 Indices and tables
 ==================