Adds download source archive link to the README.

README.md

 * [testing](https://download.jitsi.org/testing/) ([instructions](https://jitsi.org/Main/InstallJitsiMeetDebianTestingRepository))
 * [nightly](https://download.jitsi.org/unstable/) ([instructions](https://jitsi.org/Main/InstallJitsiMeetDebianNightlyRepository))
 
+You can download source archives (produced by ```make source-package```):
+* [source builds](https://download.jitsi.org/jitsi-meet/src/)
+
 You can get our mobile versions from here:
 * [Android](https://play.google.com/store/apps/details?id=org.jitsi.meet)
 * [iOS](https://itunes.apple.com/us/app/jitsi-meet/id1165103905)