Add a note explaining the version pinning.

README.md

diff --git a/README.md b/README.md
index c80c215..1eec95f 100644
--- a/README.md
+++ b/README.md
@@ -55,6 +55,9 @@
 include: package:pedantic/analysis_options.yaml
 ```
 
+This example uses a pinned version because every added lint rule could break a
+build for projects using it.
+
 ## Unused Lints
 
 The following lints have been considered and will _not_ be enforced: