Added prerequisite instructions for CentOS 7

Signed-off-by: Mads Robin Christensen <mads@v42.dk>

README.rst

@@ -111,6 +111,14 @@ Installing prerequisites on ArchLinux::
     sudo pacman -S base-devel python2 python-pip \
                    python-setuptools python-virtualenv sqlite3
 
+Installing prerequisites on CentOS 7::
+
+    sudo yum install libtiff-devel libjpeg-devel libzip-devel freetype-devel \
+                     lcms2-devel libwebp-devel tcl-devel tk-devel \
+                     python-virtualenv
+                     sudo yum groupinstall "Development Tools"
+
+
 Installing prerequisites on Mac OS X::
 
     xcode-select --install