Instruct Android developer to use 4.2.x branch. (#260)

Author: jianjunz

README.md

@@ -45,7 +45,7 @@ target_os = []
 
 ### Build
 #### Windows
-- Set environmental variable ````BOOST_ROOT```` to your boost source tree.
+- Set environmental variable `BOOST_ROOT` to your boost source tree.
 - Run `gclient sync`. It may take a long time to download large amount of data.
 - Go to src/srcitps/ directory, and run: `python build-win.py --gn_gen --sdk --tests --ssl_root /path/to/ssl --msdk_root /path/to/msdk --output_path /path/to/out`. The built binary will be under `output_path`, the document for sdk will also be copied to this directory if docs has been generated. If not set `output_path`, the built binary will be under src/out directory. Be noted the first time you run this would take a long time to pull chromium/webrtc dependencies and require a network accessible to Google's code/storage infrastructure. `ssl_root` to the directory of your OpenSSL 1.1.0 binary. `msdk_root` to the directory of your Intel Media SDK for Windows, version 2018 R1 or higher. `gn_gen` to generate args.gn, you need to add `--gn_gen` at the first time build or when you changed `msdk_root` and `ssl_root` path.
 
@@ -57,6 +57,9 @@ target_os = []
 - Run `gclient sync`. It may take a long time to download large amount of data.
 - Build OWT iOS SDK with `scripts\build.py`.
 
+#### Android
+This branch does not support Android build, please switch to `4.2.x` branch.
+
 
 ## How to contribute
 We warmly welcome community contributions to owt-client-native repository. If you are willing to contribute your features and ideas to OWT, follow the process below:
@@ -72,5 +75,4 @@ Use the "Issues" tab on Github.
 
 ## See Also
 http://webrtc.intel.com
-