Document parallel stat calls

Needed for parity with git, which already does this. libgit2 does not.

Author: joshtriplett

README.md

@@ -252,6 +252,7 @@ become available.
   * diffing of git-object::Tree structures
   * diffing, merging, working with hunks of data
   * find differences between various states, i.e. index, working tree, commit-tree
+  * Parallel stat calls to check/update objects in index
   * [ ] API documentation with examples
 
 ### git-features