More duplicate the description about doctests

This is already described in the other two sections (top-level and
"known limitations" sections), but some users do not notice them.

Closes #244

README.md

@@ -336,6 +336,8 @@ By default, run tests (via `cargo test`), and print the coverage summary to stdo
 cargo llvm-cov
 ```
 
+Currently, doc tests are disabled by default because nightly-only features are required to make coverage work for doc tests. see [#2] for more.
+
 To run `cargo run` instead of `cargo test`, use `run` subcommand.
 
 ```sh