Add libzstd to CentOS dependencies README

Missing documentation on newly required `libzstd` dependency.
Reviewed-by: NJimmy Yih <jyih@pivotal.io>
Reviewed-by: NDaniel Gustafsson <dgustafsson@pivotal.io>

README.CentOS.bash

@@ -16,6 +16,7 @@ sudo yum install -y \
     libkadm5 \
     libyaml-devel \
     libxml2-devel \
+    libzstd-devel \
     openssl-devel \
     perl-ExtUtils-Embed \
     python-devel \