readme: Avoid false expectations by removing mention of third-party plugins

Author: hukkin

README.md

@@ -38,21 +38,6 @@ pipx install mdformat
 pipx inject mdformat mdformat-gfm
 ```
 
-Note that GitHub's Markdown renderer supports syntax extensions not included in the GFM specification.
-For full GitHub support do:
-
-```bash
-pipx install mdformat
-pipx inject mdformat mdformat-gfm mdformat-frontmatter mdformat-footnote mdformat-gfm-alerts
-```
-
-Install with [Markedly Structured Text (MyST)](https://myst-parser.readthedocs.io/en/latest/using/syntax.html) support:
-
-```bash
-pipx install mdformat
-pipx inject mdformat mdformat-myst
-```
-
 <!-- end installing -->
 
 <!-- start cli-usage -->