Cleanup README

[stv0g]

No description provided.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (30badd8) 58.83% compared to head (bb62f20) 58.83%.

❗ Current head bb62f20 differs from pull request most recent head 449d437. Consider uploading reports for the commit 449d437 to get more accurate results

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[at-wat]

Hierarchy of the sections are changed to

## Install
### Usage
### More Examples
### Available Media Inputs
### Available Codecs
#### Video Codecs
##### x264
### FAQ
#### Failed to find the best driver that fits the constraints
### Roadmap
### Community
### Contributing
### License

but I don't think these '###' sections are subsections of ## Install

[stv0g]

Oh, I missed the install section. Thanks. I fixed it now.

All Pion repos are using level-3 sections in the readme as the highest level (###).