Added pkg-config as required dependency

Summary:
Faced this problem on Ubuntu 14.04.4 LTS

```
configure:16581: error: possibly undefined macro: AC_MSG_NOTICE
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
autoreconf: /usr/bin/autoconf failed with exit status: 1
```
Fixed with `apt-get install pkg-config`
Closes https://github.com/facebook/folly/pull/579

Reviewed By: yfeldblum

Differential Revision: D4886216

Pulled By: Orvid

fbshipit-source-id: 88159b70bfcf62d01bef502b3a5da6c9d81499bc

README.md

@@ -113,7 +113,8 @@ sudo apt-get install \
     zlib1g-dev \
     binutils-dev \
     libjemalloc-dev \
-    libssl-dev
+    libssl-dev \
+    pkg-config
 ```
 
 If advanced debugging functionality is required