build: switch source archive to xz compression

And a few minor README improvements

Change-Id: I667860f46b19d1ec885fd81910a116581007ac8f

wscript

@@ -400,7 +400,9 @@ def version(ctx):
         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)