io: Announce support for enum class with non default undelying size.

Also announce increase of `TStreamerInfo` version to 10

README/ReleaseNotes/v636/index.md

@@ -69,6 +69,9 @@ The following people have contributed to this new version:
     [RNTupleMergeOptions](https://root.cern/doc/v634/structROOT_1_1Experimental_1_1Internal_1_1RNTupleMergeOptions.html));
   * "rntuple.ErrBehavior=(Abort|Skip)": RNTuple-specific option that specifies the behavior of the RNTupleMerger on error (see link above);
   * "rntuple.ExtraVerbose": RNTuple-specific option that tells the RNTupleMerger to emit more information during the merge process.
+* Added support for `enum class` with a non default underlying size, for example `enum smallenum: std::int16_t`.  The default is 32 bits.  All enums, independently of their in memory size are stored on file using 32 bits to enable forward compatibility of the file; files created with a `enum class` with a non default underlying size can be read with old version of ROOT into a `enum` type of default size.
+* * Note: `enum class` with an underlying size strictly greater than 32 bits are not supported since they would be truncated when stored on file.
+* The version number of `TStreamerInfo` has been increase to 10 to encoded the addition of the support for `enum class` with a non default underlying size. This allows the opportunity to detect files written by old version of ROOT (`v9` and older of `TStreamerInfo`) where  `enum class` with a non default underlying size where stored incorrectly but recoverably.  Those files can be recover by using I/O customization rules that takes in consideration their size at the time of writing (this information is not recorded in the `ROOT` file).  See https://github.com/root-project/root/pull/17009#issuecomment-2522228598 for some examples.
 * New attribute for I/O customization rules: `CanIgnore`.  When using this attribute the rule will be ignored if the input is missing from the schema/class-layout they apply to instead of issue a `Warning`
 
 ## RDataFrame