Headline levels for generated content not semantic

[dkundel]

Hi there,

If you run oclif-dev readme it currently generates a bunch of h2 tags for the commands using ## but if you put it in a context of a README.md file that's ideally semantically correct there should only be one h1 which means that if we would nest the commands under a Commands headline it should ideally be h3. For example:

# oclif-dev CLI
## Usage
### `oclif-dev readme`

Rendered output:

oclif-dev CLI

Usage

oclif-dev readme

[grempe]

This is a very helpful command, but it is marred by this issue.

The Usage and Commands data that is inserted needs to be configurable to work with different semantic heading levels as appropriate for the project's README.md.

The example should be modified to not suggest # Usage and # Commands and the generated sub-headings should not assume that ## is the right heading level for every README.