update readme with supported/unsupported jmespath constructs

[kbroughton]

It would be nice to know in advance what parts of the jmespath spec are supported.

jq allows top level dots. So
echo '{"foo": "bar"}' | jq '.foo
bar

Also echo '{"a": "b", "c": "d"}' | jq '[.a, .c]' works in jq.

But jmespath parser complains that the '.' is an invalid character. This seems to be only the case when it starts a pattern expression.