docs: clarify comment about obsolete prefixes (#749)

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

---------

Co-authored-by: Jordan Harband <ljharb@gmail.com>

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.
+Support for stripping a leading "v" is kept for compatibility with `v1.0.0` of the SemVer
+specification but should not be used anymore.
 
 ## Ranges