Doc: Update PyXB version in README.md
Doc: Update PyXB version in README.md

Update PyXB version to 1.2.6 in README, and add a link to resource
for compiling MADlib from source on Linux.

Closes #212

njayaram2/madlib,madlib/madlib,madlib/madlib,orhankislal/madlib,apache/incubator-madlib,apache/incubator-madlib,orhankislal/madlib,njayaram2/madlib,orhankislal/madlib,njayaram2/incubator-madlib,iyerr3/incubator-madlib,hpandeycodeit/incubator-madlib,iyerr3/incubator-madlib,iyerr3/incubator-madlib,madlib/madlib,orhankislal/madlib,njayaram2/madlib,njayaram2/incubator-madlib,orhankislal/incubator-madlib,iyerr3/incubator-madlib,orhankislal/madlib,apache/incubator-madlib,iyerr3/incubator-madlib,orhankislal/incubator-madlib,hpandeycodeit/incubator-madlib,madlib/madlib,njayaram2/incubator-madlib,apache/incubator-madlib,njayaram2/incubator-madlib,hpandeycodeit/incubator-madlib,orhankislal/incubator-madlib,hpandeycodeit/incubator-madlib,hpandeycodeit/incubator-madlib,njayaram2/madlib,njayaram2/madlib,orhankislal/incubator-madlib,orhankislal/incubator-madlib,njayaram2/incubator-madlib,apache/incubator-madlib,madlib/madlib