Update README.md (facebook#1268)

Summary:
I tried building on a fresh docker image ubuntu:16.04, the current setup doesn't work because libunwind-dev is missing. Adding this fixed the problem for me.
Pull Request resolved: facebook#1268

Reviewed By: Orvid

Differential Revision: D18759832

Pulled By: yfeldblum

fbshipit-source-id: 8a861844e0b4687bf0107553274322954fdc1c00

README.md

@@ -132,7 +132,8 @@ sudo apt-get install \
     binutils-dev \
     libjemalloc-dev \
     libssl-dev \
-    pkg-config
+    pkg-config \
+    libunwind-dev
 ```
 
 Folly relies on [fmt](https://github.com/fmtlib/fmt) which needs to be installed from source.