Update documentation on pluggable Http Client

[ErrorxCode]

The Library uses Java HttpClient, which was removed in Android 7. Using this library on > Android 7 results in the following error:

NoClassDefFoundError: Failed resolution of: Ljava/net/http/HttpClient;

To Reproduce
Run GitHub.connectUsingOAuth() on android (API > 24)

Expected behaviour
No error, connection established

Desktop (please complete the following information):

• OS: Android
• Version 16

Please migrate from java HttpClient to OkHttp to keep wider support

[bitwiseman]

This library already supports okhttp3 via OkHttpGitHubConnector.

Create a OkHttpGitHubConnector and pass it to GithubBuilder.withConnector().

The docs need updating to make this easier to find and correct for version 2.x.