Add caveat for covering features

This section explains the caveats and possible solutions for features, detailing the tooling needed to 100% cover such a project.

The need for this PR is based on a conversation with @Eh2406.

Moved this PR here based on @Eh2406's comment: rust-lang/reference#1195 (comment)

Author: yoav-lavi

src/doc/src/reference/features.md

@@ -513,3 +513,9 @@ source and inspect it.
 
 [`cargo vendor`]: ../commands/cargo-vendor.md
 [cargo-clone-crate]: https://crates.io/crates/cargo-clone-crate
+
+### Caveats
+
+Features, which are a form of conditional compilation, require an exponential number of configurations and test cases to be 100% covered. By default, testing, linting, Miri, docs and others will only run on the default set of features.
+
+There are certain tools like [cargo-hack](https://github.com/taiki-e/cargo-hack) and [cargo-all-features](https://github.com/frewsxcv/cargo-all-features) that allow you to test all combinations of features which can help reduce the amount of configuraton needed.