Added notes on building libsrtp (see meetecho#2024)

README.md

@@ -90,7 +90,9 @@ The instructions for version 2.x are practically the same. Notice that the follo
 	./configure --prefix=/usr --enable-openssl
 	make shared_library && sudo make install
 
-The Janus configure script autodetects which one you have installed and links to the correct library automatically, choosing 2.x if both are installed. If you want 1.5 or 1.6 to be picked, pass `--disable-libsrtp2` when configuring Janus to force it to use the older version instead.
+Notice that the `--enable-openssl` part is _important_, as it's needed for AES-GCM support. As an alternative, you can also pass `--enable-nss` to have libsrtp use NSS instead of OpenSSL. A failure to configure libsrtp with either might cause undefined references when starting Janus, as we'd be trying to use methods that aren't there.
+
+The Janus configure script autodetects which one you have installed and links to the correct library automatically, choosing 2.x if both are installed. If you want 1.5 or 1.6 to be picked (which is NOT recommended), pass `--disable-libsrtp2` when configuring Janus to force it to use the older version instead.
 
 * *Note:* when installing libsrtp, no matter which version, you may need to pass `--libdir=/usr/lib64` to the configure script if you're installing on a x86_64 distribution.