Add manifest to include Calico 2.5

[tmjd]

Description

To upgrade Canal to use Calico v2.5 it is necessary to have dedicated manifests for Kubernetes 1.7 (and above) because 2.5 will not work with K8s <1.7. I've created the 1.7 manifests, updated the docs, and included a note linking to the Calico 2.4->2.5 upgrade proceedure.

Tests

• I did a Canal install on a setup based off the Calico K8s Vagrant install and an install in GCE (done with terraform and kubeadm) with the 1.6 manifests, ran the 2.4->2.5 upgrade, and then installed the new manifests added by this PR.
• I also did a test of installing fresh with the new manifests added by this PR. The above setups were validated by doing the Calico "Simple Policy Tutorial".

[tmjd]

@gunjan5 Do you want to take another look now that I swapped the order? @emanic Could you please take a look at the README change? Thanks

[emanic]

LGTM, thanks @tmjd ! 👍