chore: clarify comment about obsolete prefixes

Adds an explanation why the "=" and "v" prefixes should not be used anymore.

Author: mbtools

README.md

@@ -141,6 +141,8 @@ A "version" is described by the `v2.0.0` specification found at
 <https://semver.org/>.
 
 A leading `"="` or `"v"` character is stripped off and ignored.
+These prefixes are kept for compatibility with `v1.0.0` of the SemVer
+specification but should not be used anymore.
 
 ## Ranges