readme instructions are out of date

[mschoch]

We restructured the repo and now the instructions are out of date.

1. Most of the README now only applies to the bleve-bench sub-command, probably should move to a README inside that folder.
2. bleve-bench now supports running multiple configurations in a single run -- @nimishzynga can you assist in writing this portion?
3. bleve-blast command has no documentation at all -- this part should be done my @mschoch

[mschoch]

I just put together the following steps which were tested to work in clean GOPATH:

$ mkdir /tmp/gp
$ export GOPATH=/tmp/gp
$ export CGO_LDFLAGS="-L/Users/mschoch/Documents/research/rocksdb-rocksdb-3.11.2 -lrocksdb"
$ go get -tags 'forestdb rocksdb' github.com/blevesearch/bleve-bench/...
$ cd $GOPATH/src/github.com/blevesearch/bleve-bench/cmd/bleve-bench
$ go build -tags 'forestdb rocksdb'
$ ./bleve-bench -config ../../configs/rocksdb.json -source ~/go/src/github.com/blevesearch/bleve-bench/tmp/enwiki.txt -count 10000
Using Index Type: upside_down
Using KV store: rocksdb
Using KV config: map[string]interface {}{"max_open_files":200, "total_threads":4}
elapsed,docs,avg_single_doc_ms,avg_batched_doc_ms,query_water_matches,first_query_water_ms,avg_repeated5_query_water_ms
4555,1000,6.066079,4.320553,179,1.103338,0.651298
9287,2000,6.024917,4.517125,377,2.143571,1.561829
15427,3000,6.273440,6.020474,600,2.374093,1.562552
20525,4000,6.280933,4.868989,795,14.566885,6.042588
26550,5000,7.443943,5.758631,988,3.712535,3.131338
31934,6000,5.959829,5.254284,1133,2.630657,2.288392
37396,7000,7.660398,5.125245,1314,2.956244,3.301806
44120,8000,9.033230,6.332017,1543,3.370009,3.769306
50883,9000,5.449901,6.776020,1682,6.821804,6.673287
56537,10000,7.308540,5.335015,1873,9.023563,6.978587