Better docs for building/linking

[Owen Nelson]

Had a couple folks report problems getting the library to build which suggests we have a documentation gap.

Probably the thing to do is to put a block up near the top of the readme that shows

• how to depend on jq-rs
• how to specify the "bundled" feature (on jq-rs, rather than depending directly on jq-sys)
• explain that specifying "bundled" should mean you _don't need to specify JQ_LIB_DIR or install jq-related lib packages (bundled compiles jq from source by itself)
• explains that this crate will not work with jq 1.5, and requires 1.6

Also, it may be very useful to keep a Dockerfile in the repo to show the different compile options.