Update README.md to mention about adding indexes to mongo db to improve the performance

Without indexes, if you try to browse to 3rd, 4th page onward, Mongo Cursor throws a exception and asks us to add indexes so that performance is good

Adding these indexes solved the case

> db.results.ensureIndex( { 'meta.SERVER.REQUEST_TIME' : -1 } )
> db.results.ensureIndex( { 'profile.main().wt' : -1 } )
> db.results.ensureIndex( { 'profile.main().mu' : -1 } )
> db.results.ensureIndex( { 'profile.main().cpu' : -1 } )
> db.results.ensureIndex( { 'meta.url' : 1 } )

Actually index "meta.SERVER.REQUEST_TIME" is enough, however as a best practice lets add all the keys we are using in our queries