document how to get keyring_vault work on Centos6

Default curl version on Centos6 is an ancient one (7.19.7, released on November 4 2009). The default curl installation also depends on NSS library. This is known to create memory corruption issues

https://bugzilla.redhat.com/show_bug.cgi?id=1057388

Please note that this ticket was closed as wont fix and users were encouraged to use centos7. Thus PS can be unstable when linked with this curl version. We recommend using latest CURL with keyring_vault. One way of getting latest CURL is to build it from source. The source can be downloaded from CURL official website. After unpacking it should be configured to use openssl (since this is what PS is using). This can be done by using: ./configuration --with-ssl --prefix=<INSTALATION DIRECTORY> (optionally --without-nss as nss was causing issues). Then the curl needs to be build (make) and install (make install). Before you start PS make sure that this version of curl will be picked up. One way of doing it is updating LD_LIBRARY_PATH : export LD_LIBRARY_PATH=<INSTALATION DIRECTORY>/lib:$LD_LIBRARY_PATH . Detailed instructions on building CURL from source can be found here: https://curl.haxx.se/docs/install.html .