[NFC] Add MT Snapshot to RNTuple release note

Author: enirolf

README/ReleaseNotes/v638/index.md

@@ -163,6 +163,7 @@ If you want to keep using `TList*` return values, you can write a small adapter
 - The Snapshot method has been refactored so that it does not need anymore compile-time information (i.e. either template arguments or JIT-ting) to know the input column types. This means that any Snapshot call that specifies the template arguments, e.g. `Snapshot<int, float>(..., {"intCol", "floatCol"})` is now redundant and the template arguments can safely be removed from the call. At the same time, Snapshot does not need to JIT compile the column types, practically giving huge speedups depending on the number of columns that need to be written to disk. In certain cases (e.g. when writing O(10000) columns) the speedup can be larger than an order of magnitude. The Snapshot template is now deprecated and it will issue a compile-time warning when called. The function overload is scheduled for removal in ROOT 6.40.
 - Experimental support for systematic variations has been added to snapshots. Refer to the section on [systematic variations](https://root.cern.ch/doc/v638/classROOT_1_1RDataFrame.html#systematics) in the RDataFrame manual. The support currently only encompasses single-threaded snapshots to TTree, but support will be extended to multi-threaded snapshots in TTree and to RNTuple in future ROOT versions.
 - The default compression setting for the output dataset used by Snapshot has been changed from 101 (ZLIB level 1, the TTree default) to 505 (ZSTD level 5). This is a better setting on average, and makes more sense for RDataFrame since now the Snapshot operation supports more than just the TTree output data format. This change may result in smaller output file sizes for your analyses that use Snapshot with default settings. During the 6.38 development release cycle, Snapshot will print information about this change once per program run. Starting from 6.40.00, the information will not be printed. The message can be suppressed by setting ROOT_RDF_SILENCE_SNAPSHOT_INFO=1 in your environment or by setting 'ROOT.RDF.Snapshot.Info: 0' in your .rootrc.
+- Multithreaded snapshotting to RNTuple has been added. This will be used automatically when `ROOT::SetImplicitMT()` has been called and `RSnapshotOptions::fOutputFormat` has been set to `ESnapshotOutputFormat::kRNTuple`.
 
 ### Distributed RDataFrame
 - Processing of RNTuples is now done in the same way as for TTrees, as it is a more efficient way of distributing the computations. The API remains unchanged.