add vcpkg as an windows option

Summary: Closes https://github.com/facebook/rocksdb/pull/2629

Differential Revision: D5483751

Pulled By: sagar0

fbshipit-source-id: 9719ef9edd936dbb89b8988e3f4cb912a234f00e

INSTALL.md

@@ -113,6 +113,8 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
 * **Windows**:
   * For building with MS Visual Studio 13 you will need Update 4 installed.
   * Read and follow the instructions at CMakeLists.txt
+  * Or install via [vcpkg](https://github.com/microsoft/vcpkg) 
+       * run `vcpkg install rocksdb`
 
 * **AIX 6.1**
     * Install AIX Toolbox rpms with gcc