[skip ci] Merge pull request #222 from CosmWasm/code-coverage

Document using tarpaulin

README.md

@@ -163,6 +163,39 @@ If you hit any issues there and want to debug, you can try to run the
 following in each contract dir:
 `RUSTFLAGS="-C link-arg=-s" cargo build --release --target=wasm32-unknown-unknown --locked`
 
+## Quality Control
+
+One of the basic metrics of assurance over code quality is how much is covered by
+unit tests. There are several tools available for Rust to do such analysis and
+we will describe one below. This should be used as a baseline metric to give some
+confidence in the code.
+
+Beyond code coverage metrics, just having a robust PR review process with a few
+more trained eyes looking for bugs is very helpful in detecting paths the original
+coder was not aware of. This is more subjective, but looking at the relevant PRs
+and depth of discussion can give an idea how much review was present.
+
+After that, fuzzing it (ideally with an intelligent fuzzer that understands the domain)
+can be valuable. And beyond that formal verification can provide even more assurance
+(but is very time consuming and expensive).
+
+### Code Coverage
+
+I recommend the use of [tarpaulin](https://github.com/xd009642/tarpaulin): `cargo install cargo-tarpaulin`
+
+To get some nice interactive charts, you can go to the root directory and run:
+
+`cargo tarpaulin -o html`
+and then `xdg-open tarpaulin-report.html` (or just `open` on MacOS).
+
+Once you find a package that you want to improve, you can do the following to just
+analyze this package, which gives much faster turn-around:
+
+`cargo tarpaulin -o html --packages cw3-fixed-multisig`
+
+Note that it will produce a code coverage report for the entire project, but only the coverage in that
+package is the real value. If does give quick feedback for you if you unit test writing was successful.
+
 ## Licenses
 
 This repo contains two license, [Apache 2.0](./LICENSE-APACHE) and