updated readme

git-svn-id: http://svn.code.sf.net/p/chapel/code/branches/collaborations/futures@23765 3a8e244f-b0f2-452b-bcba-4c88e055c3ca
clarified example returns an int in readme

git-svn-id: http://svn.code.sf.net/p/chapel/code/branches/collaborations/futures@23766 3a8e244f-b0f2-452b-bcba-4c88e055c3ca
future coercion example in readme

git-svn-id: http://svn.code.sf.net/p/chapel/code/branches/collaborations/futures@23767 3a8e244f-b0f2-452b-bcba-4c88e055c3ca