Provide a tutorial guide for how to write OAM spec

[hongchaodeng]

As I'm reviewing the PR https://github.com/crossplaneio/crossplane/pull/1132 from CrossPlane I found that it is very difficult to jump through various pages in this repo to find examples of OAM types (AppConfig, Trait, etc.).

To empathize with OAM users and the future ourselves, we'd better provide a tutorial guide putting together yaml examples of OAM concepts in one place. Similar to the introduction doc but should be simpler and full of yaml examples.