Write code coverage instructions to CONTRIBUTING.md

This should help everyone get quickly started on test code coverage measurement
using Tarpaulin.

Signed-off-by: Paul Mabileau <[email protected]>

Author: PaulDance

CONTRIBUTING.md

@@ -161,6 +161,42 @@ requires an extra flag which is documented below.
   * Animations are **disabled** during tests, so setting the speed will have no
     impact on anything. You should not write unit tests or doctests that depend
     on animations.
+* To measure test code coverage, several steps:
+  * Before anything, we use [Tarpaulin](https://github.com/xd009642/tarpaulin)
+    in order to run the coverage. As of writing this, Tarpaulin only supports the
+    **Linux x86_64** platform, so make sure to be using a compatible setup.
+  * First, install Tarpaulin using `cargo install cargo-tarpaulin --version 0.13.3`.
+    * We are currently using not the most recent version of Tarpaulin because
+      one of our indirect dependencies itself uses a Tarpaulin configuration
+      attribute refactored in `0.13.4`, which causes a compilation error. So
+      until this indirect dependency gets to a newer version either by a direct
+      dependency or us requiring it, this is how it will be.
+    * This will very probably change in the future, in that case, it will be
+      properly documented here and one will be able to omit the `--version 0.13.3`
+      in order to download and install the latest version.
+  * Then run `cargo +nightly tarpaulin --features "test unstable" --run-types Tests Doctests`
+    * `--features "test unstable"` is required for the same reasons as the tests:
+      the documentation tests use the `test` feature in order to get configured
+      automatically and many tests require `unstable` features.
+    * `+nightly` has to be given because Tarpaulin runs documentation tests only
+      on the nightly channel. If you don't have it installed yet, please refer to
+      the [rustup documentation](https://rust-lang.github.io/rustup/concepts/channels.html)
+      in order to get it on your machine.
+
+    * `--run-types Tests Doctests` is needed because Tarpaulin only measures
+      coverage from the calls generated by the public tests by default, so we
+      have to explicitely specify documentation tests as well: a lot of
+      documentation tests have been implemented, so taking them into account
+      drastically changes the overall score obtained.
+  * Tarpaulin has a lot of other useful functionalities, so be sure to check
+    them out by listing its available options using `cargo tarpaulin --help` and
+    reading [their README](https://github.com/xd009642/tarpaulin#tarpaulin),
+    especially for the `--out` option which lets you export results to many
+    different formats.
+  * When implementing new features or completing some for this project, try to
+    also write as many tests as possible in order to keep your features in
+    check and let everyone have a greater trust in them. The higher the score,
+    usually the better!
 * To generate the project's documentation, run `cargo doc`
   * To open the generated documentation, run `cargo doc --open`
   * When writing documentation, it is often easier to have the documentation