Readme update - Tumblr is OAuth 1.0

Tumblr is OAuth 1.0 https://github.com/Lusitanian/PHPoAuthLib/tree/master/src/OAuth/OAuth1/Service

Not found on the OAuth2 dir https://github.com/Lusitanian/PHPoAuthLib/tree/master/src/OAuth/OAuth2/Service
Merge pull request #87 from thomaswelton/patch-1

Readme update - Tumblr is OAuth 1.0