doc: add pkg-config as a prerequisite

Change-Id: I820491e6441e0d459834a3f47a3ca5b5516cbc04
diff --git a/INSTALL.md b/INSTALL.md
index 40fea67..91375c7 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -1,12 +1,11 @@
 
-Prerequisites
--------------
+## Prerequisites
 
-- Boost libraries >= 1.42
-- NDN-CPP-dev library (https://github.com/cawka/ndn-cpp)
+* Boost libraries >= 1.42
+* [NDN-CPP-dev library](https://github.com/cawka/ndn-cpp)
+* `pkg-config`
 
-Build
------
+## Build
 
 The following commands should be used to build NFD: