Add NEWSLETTER item about _Float16 support (HDFGroup#4197)

release_docs/NEWSLETTER.txt

@@ -1,5 +1,4 @@
 INTRODUCTION
-============
 
 This purpose of this document is to contain entries that can be used to quickly
 produce a release newsletter. When something is added to the library that is
@@ -33,3 +32,13 @@ memory leak fixes, etc. do not require entries.
     current szip and libaec options only work for building the libaec library.
 
 
+* Added support for _Float16 16-bit floating-point datatype
+
+  Support for the 16-bit floating-point _Float16 C type has been added to
+  HDF5. On platforms where this type is available, this can enable more
+  efficient storage of floating-point data when an application doesn't
+  need the precision of larger floating-point datatypes. It can also allow
+  for improved performance when converting between 16-bit floating-point
+  data and data of another HDF5 datatype.
+
+  (GitHub #4065, #2154)