[Mango query] Show warning when no matching index found.

[tiaod]

https://github.com/djc/couchdb-python/blob/e8eab1f66fdb7f82273e0780282389d512a8f519/couchdb/client.py#L812

When run a mango query without index, couchdb will return:

HTTP/1.1 200 OK
Cache-Control: must-revalidate
Content-Type: application/json
Date: Thu, 01 Sep 2016 17:25:51 GMT
Server: CouchDB (Erlang OTP)
Transfer-Encoding: chunked

{
    "warning":"no matching index found, create an index to optimize
    query time",
    "docs":[
    ]
}

http://docs.couchdb.org/en/2.1.1/api/database/find.html#creating-selector-expressions

[djc]

Thanks for reporting this. Unfortunately I no longer have time to spend on CouchDB-Python, so until someone else steps up to maintain it, there will not be a fix for this.