[doc] Add documentation for Bazel RISC-V toolchain

Signed-off-by: James Wainwright <[email protected]>

SUMMARY.md

@@ -415,6 +415,7 @@
 
 - [Build Software](./sw/doc/build_software.md)
   - [External dependencies](./third_party/README.md)
+  - [RISC-V toolchain](./toolchain/README.md)
 
 - [Device Software](./sw/device/README.md)
   - [Build & Test Rules](./rules/opentitan/README.md)

toolchain/README.md

@@ -0,0 +1,31 @@
+# OpenTitan RISC-V toolchain
+
+This directory contains the Bazel configuration for OpenTitan's RISC-V
+toolchain.
+
+This LLVM toolchain comes from the [lowrisc-toolchains] repository. See
+`third_party/lowrisc/BUILD.lowrisc_toolchain.bazel` for changing the toolchain
+version.
+
+[lowrisc-toolchains]: https://github.com/lowRISC/lowrisc-toolchains
+
+## Configuration
+
+There are four rules used to configure the toolchain:
+
+1. `cc_toolchain`: groups flags, features, and tools into a toolchain.
+2. `cc_tool_map`: assigns tools to actions.
+3. `cc_args`: defines flags to add to tools based on actions.
+3. `cc_feature`: allows `cc_args` flags to be conditionally enabled.
+
+To add new flags to a tool in the toolchain, define a new `cc_args` target
+and assign it to some actions (e.g. compiling C code, linking, etc.). Add the
+new flags to `cc_toolchain.args`.
+
+To make flags optional, define a new `cc_feature` for those `cc_args`. Features
+can be enabled at the command line using `bazel --features=$feature_name`. Add
+the flags to `cc_toolchain.known_features` and optionally to
+`cc_toolchain.enabled_features`.
+
+Bazel has three built-in features called `dbg`, `fastbuild`, and `opt` that can
+be used to enable and disable flags at different optimization levels.