Add info on how to build experimental binaries

..also update the text that says "TO.BE.DETERMINED" to the actual URL
where the experimental nightlies are published.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

experimental/README.md

@@ -11,7 +11,10 @@ please feel free to provide any feedback on these features you wish.
 
 ## Install Docker experimental
 
-Unlike the regular Docker binary, the experimental channels is built and updated nightly on TO.BE.ANNOUNCED. From one day to the next, new features may appear, while existing experimental features may be refined or entirely removed.
+Unlike the regular Docker binary, the experimental channels is built and
+updated nightly on https://experimental.docker.com. From one day to the
+next, new features may appear, while existing experimental features may be
+refined or entirely removed.
 
 1. Verify that you have `curl` installed.
 
@@ -59,6 +62,13 @@ After downloading the appropriate binary, you can follow the instructions
 >
 > 2) You can get the compressed binaries by appending .tgz to the URLs
 
+### Build an experimental binary
+You can also build the experimental binary from the standard development environment by adding
+`DOCKER_EXPERIMENTAL=1` to the environment where you run `make` to build Docker binaries. For example,
+to build a dynamically-linked Docker binary with the experimental features enabled:
+
+        $ DOCKER_EXPERIMENTAL=1 make dynbinary
+
 ## Current experimental features
 
 * [Network plugins](plugins_network.md)