Add data channel option to the readme (#2099)

* Add data channel option to the readme

* Address comments

Author: sirknightj

README.md

@@ -144,6 +144,7 @@ You can pass the following options to `cmake ..`:
 
 * `-DBUILD_SAMPLE` -- Build the sample executables. ON by default.
 * `-DIOT_CORE_ENABLE_CREDENTIALS` -- Build the sample applications using IoT credentials. OFF by default.
+* `-DENABLE_DATA_CHANNEL` -- Build SDK & samples with data channel. ON by default.
 * `-DBUILD_STATIC_LIBS` -- Build all KVS WebRTC and third-party libraries as static libraries. Default: OFF (shared build).
 * `-DADD_MUCLIBC`  -- Add -muclibc c flag
 * `-DBUILD_DEPENDENCIES` -- Whether or not to build depending libraries from source