Fix example callback route to be POST, not GET (in Readme).

[arnab]

• tiny change in README to update the route from get to post.

[coveralls]

Coverage remained the same when pulling 5c1dc01 on arnab:fix-route-example-in-readme into 7c97b20 on intridea:master.

[mbleigh]

The endpoint may be GET or POST based on the provider, and I'd wager that it's more often GET than POST (Twitter, Facebook, GitHub, LinkedIn, Google are all GET).

I'd be open to a commit that clarifies that this varies based on the strategy.