README: count the plugins the tree has, and point somewhere useful

It claimed thirteen bundled plugins and listed thirteen; there are fifteen, and
the two missing ones - backups and tools - were undiscoverable from the first
page anyone reads. The count is gone rather than corrected, because a number
typed here goes stale exactly the way it just did; the generated map carries it.

The version and count guard now covers README.md and CONTRIBUTING.md, not just
docs/. A rule that leaves the most-read file in the repository outside it is
most of a rule.

Also names the reference site as the reference site rather than by plant, adds a
pointer to START-HERE, and corrects the distribution paragraph: it told a reader
of the public mirror that the code is only available on an internal server,
which is both wrong and unhelpful now that the installer ships as a release
asset.

tests/test_docs_versions.py

@@ -40,7 +40,19 @@ PLUGIN_COUNT = re.compile(r'\b(\d+)\s+bundled plugins\b', re.I)
 
 
 def documentation_files():
-    return sorted(p for p in DOCS.rglob('*.md') if p.name not in EXEMPT)
+    """Everything published that a reader treats as documentation.
+
+    README.md and CONTRIBUTING.md are included deliberately: README is the first
+    page anyone sees on the mirror, and it was claiming thirteen bundled plugins
+    when there were fifteen. A rule that covers only docs/ leaves the most-read
+    file in the repository outside it.
+    """
+    files = [p for p in DOCS.rglob('*.md') if p.name not in EXEMPT]
+    for name in ('README.md', 'CONTRIBUTING.md'):
+        top = REPO / name
+        if top.is_file():
+            files.append(top)
+    return sorted(files)
 
 
 def current_versions():