Add more security sensitive examples to intro to illustrate suitability

[tlodderstedt]

A bit about OAuth adoption in scenarios beyond its original scope (e.g. Open Banking).

[aaronpk]

With OAuth, an end user (resource owner) can grant a printing service (client) access to their protected photos stored at a photo- sharing service (resource server), without sharing their username and password with the printing service. Instead, they authenticate directly with a server trusted by the photo-sharing service (authorization server), which issues the printing service delegation- specific credentials (access token).

[aaronpk]

Related to #168

[aaronpk]

Examples:

• granting access to account information to a financial management app.
• granting a third party app access to post to a social media account