Updated instructions for Pharo a bit

Author: mumez

doc/HowToInstall.md

@@ -27,6 +27,15 @@ Installer squeaksource
 
 You can use Metacello:
 
+```smalltalk
+Metacello new
+  baseline: 'MessagePack';
+  repository: 'github://msgpack/msgpack-smalltalk/repository';
+  load.
+```
+
+If you prefer development branch:
+
 ```smalltalk
 Metacello new
   baseline: 'MessagePack';