Update about tcmalloc library on release notes

Author: Shwetha-Acharya

docs/release-notes/10.0.md

@@ -22,6 +22,9 @@ A full list of bugs that have been addressed is included further below.
 
 - Major performance improvement of ~20% w.r.t small files
   as well as large files testing in controlled lab environments [#2771](https://github.com/gluster/glusterfs/issues/2771)
+ 
+  **NOTE**: The above improvement requires tcmalloc library to be enabled for building.  We have tested and verified tcmalloc in X86_64 platforms and is enabled   only for x86_64 builds in current release. 
+  
 - Randomized port selection for bricks, improves startup time [#786](https://github.com/gluster/glusterfs/issues/786)
 - Performance improvement with use of readdir instead of readdirp in fix-layout [#2241](https://github.com/gluster/glusterfs/issues/2241)
 - Heal time improvement with bigger window size [#2067](https://github.com/gluster/glusterfs/issues/2067)