Add instructions for creating Vault Secret ID / GitHub Token (elastic/x-pack-elasticsearch#761)
Add instructions for creating Vault Secret ID / GitHub Token (elastic/x-pack-elasticsearch#761)

The instructions over in elastic/infra are more geared toward the infra team.

Original commit: elastic/x-pack-elasticsearch@1c5125d9bc68a4a1d0794afcd70dd42e0f4df770

HonzaKral/elasticsearch,gfyoung/elasticsearch,uschindler/elasticsearch,strapdata/elassandra,gingerwizard/elasticsearch,strapdata/elassandra,vroyer/elassandra,nknize/elasticsearch,gfyoung/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,strapdata/elassandra,GlenRSmith/elasticsearch,coding0011/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,vroyer/elassandra,coding0011/elasticsearch,vroyer/elassandra,uschindler/elasticsearch,gingerwizard/elasticsearch,strapdata/elassandra,nknize/elasticsearch,nknize/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,robin13/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,HonzaKral/elasticsearch,gfyoung/elasticsearch,strapdata/elassandra,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch