[DOCS] add REST API and DESIGN doc links to readme
[DOCS] add REST API and DESIGN doc links to readme

Original commit: elastic/x-pack-elasticsearch@828d7370b0dce8a7e7c38fb9aebc503d376e3fb3

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