Update README.md dev setup instructions
Update README.md dev setup instructions

- Move Conan instructions above manual orca setup instructions
  so that people don't do manual work first (like we did).
- Inline recommendation to parallelize make with -j8.
- Fix formatting of code blocks in README.md

See issues:
 - https://github.com/greenplum-db/gpdb/issues/3178
 - https://github.com/greenplum-db/gpdb/pull/3332



Chibin/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,Chibin/gpdb,lisakowen/gpdb,greenplum-db/gpdb,Chibin/gpdb,Chibin/gpdb,edespino/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,edespino/gpdb,jmcatamney/gpdb,50wu/gpdb,jmcatamney/gpdb,Chibin/gpdb,jmcatamney/gpdb,adam8157/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,yuanzhao/gpdb,edespino/gpdb,ashwinstar/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,Chibin/gpdb,Chibin/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,jmcatamney/gpdb,edespino/gpdb,xinzweb/gpdb,50wu/gpdb,yuanzhao/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,edespino/gpdb,Chibin/gpdb,Chibin/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,edespino/gpdb,yuanzhao/gpdb,50wu/gpdb,ashwinstar/gpdb,edespino/gpdb,edespino/gpdb,lisakowen/gpdb,adam8157/gpdb,yuanzhao/gpdb,edespino/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,Chibin/gpdb,yuanzhao/gpdb,lisakowen/gpdb,50wu/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,edespino/gpdb,ashwinstar/gpdb,jmcatamney/gpdb