diff --git a/README.md b/README.md
index 7e8fd31..8b04b05 100644
--- a/README.md
+++ b/README.md
@@ -16,6 +16,36 @@ Usage & Instructions
   priority over meta.
 - Set `EXTERNAL_TOOLCHAIN = "/path/to/your/sourcery-g++-install"` in `conf/local.conf`.
 
+Optional functionality:
+
+- If the user chooses to, they may optionally decide to rebuild the Sourcery G++ eglibc
+  from source, if they have downloaded the corresponding source archive from Mentor
+  Graphics. To so, set `TCMODE = "external-sourcery-rebuild-libc"`, rather than relying
+  on the default value of `external-sourcery`.
+
+Description of Behavior
+-----------------------
+
+The meta-sourcery layer.conf automatically defines `TCMODE` for us, so this is no longer
+necessary.  The tcmode performs a number of operations:
+
+- Sets `TARGET_PREFIX` appropriately, after determining what prefix is in use by the toolchain
+- Sanity checks `EXTERNAL_TOOLCHAIN`: does the path exist? does the expected sysroot exist?
+- Sanity checks execution of the toolchain binaries
+- Sets preferences so that the `external-sourcery-toolchain` recipe is used in preference
+  to rebuilding various things from source with their own recipes
+- Extracts version information from the toolchain (e.g. by running `${TARGET_PREFIX}gcc -v`),
+  for use in the `external-sourcery-toolchain` recipe and its binary packages
+- Symlinks the toolchain binaries into the toolchain portion of the sysroot. This is done
+  in preference to adding the toolchain path to the `PATH`, to avoid the aforementioned
+  ia32 issue, and to let us work around certain issues (For example, we create an `ld.bfd`
+  link which the kernel build expects, but isn't shipped with the toolchain)
+- Adds the external toolchain `PATH` to the setup script emitted when building SDKs (e.g.
+  when bitbaking meta-toolchain)
+- Sets `GCCVERSION` to the gcc version of the toolchain, to prefer a matching gcc version for
+  the target package, if possible. Certain versions of gcc have trouble being built by other
+  versions of gcc, so this can avoid such issues.
+
 Contributing
 ------------