Add information about LPCNET_DISABLE to README.

README.md

@@ -57,7 +57,24 @@ This document describes how to build the FreeDV GUI program for various operatin
  ```
  $ ./build_linux/src/freedv
  ```
-
+
+## Building without LPCNet
+
+In preparation for possible future deprecation of FreeDV 2020 and 2020B modes, it is
+possible to build without requiring the [LPCNet](https://github.com/drowe67/LPCNet.git) library.
+To do this, pass `LPCNET_DISABLE=1` as an environment variable to the build script, i.e.
+
+```
+$ LPCNET_DISABLE=1 ./build_linux.sh
+```
+
+or alternatively, do not pass in `LPCNET_BUILD_DIR` to `cmake` if manually executing the build.
+This also has the side effect of disabling 2020 and 2020B in the user interface, preventing it
+from being selected.
+
+*Note: if you don't already have Codec2 installed on your machine, you will need to pass `-DBOOTSTRAP_LPCNET=1`
+to `cmake` in order for LPCNet to also be built.*
+
 ## Installing on Linux
 
 You need to install the codec2 and lpcnetfreedv shared libraries, and freedv-gui: