[HEAD] Add doc and update CHANGELOG.

CHANGELOG.md

-## 0.8.1 (unreleased)
+## 0.9.0 (unreleased)
 
 [CocoaPods](https://github.com/CocoaPods/CocoaPods/compare/0.8.0...master) • [Xcodeproj](https://github.com/CocoaPods/Xcodeproj/compare/0.2.3...master)
 
 ###### Enhancements
 
+- Force downloading the ‘bleeding edge’ version of a pod.
 - Support for weak frameworks.
 
 ###### Bug fixes

lib/cocoapods/podfile.rb

@@ -295,16 +295,22 @@ module Pod
     # * `<= 0.1`   Version 0.1 and any lower version
     # * `~> 0.1.2` Version 0.1.2 and the versions upto 0.2, not including 0.2
     #
-    #
-    # Finally, a list of version requirements can be specified for even more fine
+    # A list of version requirements can be specified for even more fine
     # grained control.
     #
+    #
     # For more information, regarding versioning policy, see:
     #
     # * http://semver.org
     # * http://docs.rubygems.org/read/chapter/7
     #
     #
+    # Finally, instead of a version, you can specify the `:head` flag. This will
+    # use the pod’s latest version spec version, but force the download of the
+    # ‘bleeding edge’ version. Use this with caution, as the spec might not be
+    # compatible anymore.
+    #
+    #
     # ## Dependency on a library, outside those available in a spec repo.
     #
     # ### From a podspec in the root of a library repo.