Explain the purpose of the query-grammar subcrate

Since query-grammar has been moved to a separate crate, we should explain what it's responsible for and how it fits with the rest of the library architecture.

We also moved the recursion_limit to this subcrate, the value of 100 is still prone to failure, which we should test https://github.com/tantivy-search/tantivy/blob/master/query-grammar/src/lib.rs#L1

• Add a test for recursion limit in query-grammar
• Add a Readme to query-grammar to explain what it does
• Edit query-grammar/Cargo.toml to reflect correct keywords etc