Add instructions for code coverage (#30)

Closes #28

Reviewed-on: https://git.wojciechkozlowski.eu/wojtek/musichoard/pulls/30

README.md

@@ -0,0 +1,32 @@
+# Music Hoard
+
+## Code Coverage
+
+### Pre-requisites
+
+``` sh
+rustup component add llvm-tools-preview
+cargo install grcov
+```
+
+### Generating Code Coverage
+
+```sh
+cargo clean
+env RUSTFLAGS="-C instrument-coverage" \
+    LLVM_PROFILE_FILE="target/debug/profraw/musichoard-%p-%m.profraw" \
+    cargo test
+grcov target/debug/profraw \
+      --binary-path ./target/debug/ \
+      --output-types html \
+      --source-dir . \
+      --ignore-not-existing \
+      --ignore "tests/*" \
+      --ignore "src/main.rs" \
+      --excl-start "#\[cfg\(test\)\]" \
+      --output-path ./target/debug/coverage/
+xdg-open target/debug/coverage/index.html
+```
+
+Note that some changes may not be visible until `target/debug/coverage` is removed and the `grcov`
+command is rerun.