Update Custom Scalar version naming guidelines

Clarified naming convention for new Custom Scalar versions.

Author: martinbonnin

README.md

@@ -55,9 +55,8 @@ Specification semantics must not change, as specifications are publicly
 available reference documents. We will permit small edits which do not change
 specification semantics, such as typo fixes.
 
-A new version of a Custom Scalar specification should have a new scalar name,
-either with a number appended such as `<scalar-name>2`, or a different name such
-as `OffsetDateTime` to differentiate from `DateTime`.
+A new version of a Custom Scalar specification can either use a new scalar name
+or aadd a version as part of its url like `scalars/contributed/<github-user-name>/<version>/<scalar-name>.md`.
 
 ## Licensing