Reduce size of dctx by reutilizing dst buffer (#2751)

* Reduce size of dctx by reutilizing dst buffer

Co-authored-by: Binh Vo <binhvo@fb.com>

lib/README.md

@@ -155,6 +155,12 @@ The file structure is designed to make this selection manually achievable for an
 - The build macro `ZSTD_NO_INTRINSICS` can be defined to disable all explicit intrinsics.
   Compiler builtins are still used.
 
+- The build macro `ZSTD_LITBUFFEREXTRASIZE` can be set to control the amount of extra memory used
+  during decompression to store literals.  This defaults to 64kB.  Reducing it can reduce the
+  memory footprint required for decompression by increasing the portion of the literal buffer that
+  is stored in the unwritten portion of the dst buffer, at the cost of performance impact for
+  decompression.
+
 
 #### Windows : using MinGW+MSYS to create DLL