readme: add a section about CONFIG

Change-Id: I5a6cd28ee9a3b37921355af46dafdd0f4b606c1a
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>

README.md

@@ -69,6 +69,27 @@ FreeBSD:
 
     4) (cd dpdk-16.11 && gmake install T=x86_64-native-bsdapp-clang DESTDIR=.)
 
+Build Configuration
+===================
+
+Optional components and other build-time configuration are controlled by the `CONFIG` file
+in the root SPDK directory.  `CONFIG` is a Makefile fragment that may be edited before building to
+control which options are enabled.
+
+Boolean (on/off) options are configured with a 'y' (yes) or 'n' (no).  For example, this line of
+`CONFIG` controls whether the optional RDMA (libibverbs) support is enabled:
+
+    CONFIG_RDMA?=n
+
+To enable RDMA, this line of CONFIG may be modified to contain 'y' instead of 'n'.
+
+Alternatively, `CONFIG` options may also be overrriden on the `make` command line:
+
+    make CONFIG_RDMA=y
+
+The options specified on the `make` command line take precedence over the default values in
+`CONFIG`.
+
 Building
 ========