docs(readme): reflect completed MyST migration

The MyST migration is complete: source_suffix in docs/conf.py is now
['.md'] only, all canonical pages are .md, and the pre-migration RST
originals are archived under docs/_rst_legacy/ (excluded from the
build). Update the README accordingly:

- Add a 'Source format' section stating pages are MyST Markdown and
  pointing at the _rst_legacy/ archive so newcomers know not to edit
  it. Mention that the VyOS command directives are written as MyST
  fenced blocks via myst_fence_as_directive, and that docs/_include/
  *.txt snippets stay RST because cmdincludemd parses their content
  as RST (per 9c815d6).
- Drop the stale 'MyST Markdown for migrated pages, RST for the rest'
  line from the Contributing section; rephrase to point at the actual
  topics AGENTS.md covers today (MyST conventions, command directives,
  linter suppression markers, Copilot/CodeRabbit workflow).

1 files changed, 17 insertions, 3 deletions

diff --git a/README.md b/README.md
index 290c6ce1..17ddf8a6 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,19 @@ https://docs.vyos.io.
 The earlier wiki for VyOS 1.1.x and pre-1.2.0 docs is preserved on the
 [Wayback Machine](https://web.archive.org/web/20200225171529/https://wiki.vyos.net/wiki/Main_Page).
 
+## Source format
+
+Pages are [MyST Markdown](https://myst-parser.readthedocs.io/) (`.md`) and
+build with Sphinx — `source_suffix` in `docs/conf.py` lists `.md` only.
+The pre-migration RST originals are archived under `docs/_rst_legacy/`
+for reference; they are excluded from the build and should not be edited.
+
+VyOS-specific command directives (`cfgcmd`, `opcmd`, `cmdincludemd`) are
+written as MyST fenced blocks and rendered as directives via
+`myst_fence_as_directive`. Shared snippets under `docs/_include/*.txt` are
+still RST — they are included into MyST pages through `cmdincludemd`, which
+parses their content as RST so the legacy templates keep working unchanged.
+
 ## Branches
 
 The documentation repository tracks the same branch convention as the VyOS
@@ -60,6 +73,7 @@ Output lands in `docs/_build/html/`.
 
 ## Contributing
 
-See [AGENTS.md](AGENTS.md) for the full contributor guide — source format
-conventions (MyST Markdown for migrated pages, RST for the rest), CLI
-directive syntax, IP-address rules, the linter, and the bot review workflow.
+See [AGENTS.md](AGENTS.md) for the full contributor guide — MyST source
+conventions, the VyOS command directives (`cfgcmd` / `opcmd` /
+`cmdincludemd`), IP-address rules, the linter and its suppression markers,
+and the Copilot/CodeRabbit bot review workflow.