build: switch source archive to xz compression

Mention gerrit in the README

Change-Id: I468cedde1a15f760aef00ac2995dd5f73728b89e
diff --git a/wscript b/wscript
index fd0320f..8d46c71 100644
--- a/wscript
+++ b/wscript
@@ -177,7 +177,9 @@
         Logs.warn('%s is not writable (%s)' % (versionFile, e.strerror))
 
 def dist(ctx):
+    ctx.algo = 'tar.xz'
     version(ctx)
 
 def distcheck(ctx):
+    ctx.algo = 'tar.xz'
     version(ctx)