Add a guide on extending Pedestal via its protocols

[mtnygard]

Some of the interesting protocols to discuss:

1. IntoInterceptor
2. Interceptor
3. RouterSpecification

[mtnygard]

4. WriteableBody
5. WriteableBodyAsync

[ddeaguiar]

Note to self: Let's revisit what reference material exists for this protocols. I'm hesitant to create full blown guides for them.