add just a few docs (issue #61)

[droundy]

I'm figuring I'll probably add more as I work through this, and could either keep updating this PR or add more, depending on whether you'd prefer to merge more smaller PR or a larger one later.

[loyd]

Thanks for MR

[loyd]

JFYI:

warning: unknown attribute `norun`. Did you mean `no_run`?
  --> /home/workspace/fave/clickhouse.rs/src/query.rs:48:5
   |
48 | /     /// Executes the query, returning a [`RowCursor`] to obtain results.
49 | |     ///
50 | |     /// # Example
51 | |     /// ```norun
...  |
64 | |     /// }
65 | |     /// ```
   | |___________^
   |
   = help: the code block will either not be tested if not marked as a rust one or will be run (which you might not want)
   = note: `#[warn(rustdoc::invalid_codeblock_attributes)]` on by default

Also it doesn't compile.

Fixed in master