Adds documentation on Devfile file reference

**What type of PR is this?**
> Uncomment only one ` /kind` line, and delete the rest.
> For example, `> /kind bug` would simply become: `/kind bug`

> Documentation changes: Please include [skip ci] in your commit message as well
/kind documentation
[skip ci]

**What does does this PR do / why we need it**:

This PR adds documentation regarding each section of Devfile and what we
support / do not support

**Which issue(s) this PR fixes**:

N/A

**How to test changes / Special notes to the reviewer**:

N/A

Signed-off-by: Charlie Drage <charlie@charliedrage.com>