Added variables to build bindings when using VCPKG (#440)

Summary:
**Original Issue**: flashlight/flashlight#437

Missing documentation for building python bindings when using VCPKG to build flashlight

### Test Plan (required)
None, documentation change only

Pull Request resolved: flashlight/flashlight#440

Reviewed By: avidov

Differential Revision: D26106659

Pulled By: tlikhomanenko

fbshipit-source-id: e9bf3eb18444f28d354631c9ffa7829f52b888b9

bindings/python/README.md

@@ -55,6 +55,12 @@ cd bindings/python
 python3 setup.py install
 ```
 
+In case you used VCPKG to build flashlight, you also need to declare these variables before running setup.py:
+```
+export CMAKE_LIBRARY_PATH=[vcpkg_installation_location]/installed/x64-linux/lib
+export CMAKE_INCLUDE_PATH=[vcpkg_installation_location]/installed/x64-linux/include
+```
+
 with `pip` either install from `pypi` (coming soon)
 ```
 pip3 install pyflashlight