docs: add instructions for test coverage

.gitignore

@@ -2,4 +2,7 @@
 .packages
 .dart_tool
 .pub
-pubspec.lock
+pubspec.lock
+
+# Coverage
+coverage/

README.md

@@ -5,3 +5,20 @@ To generate bindings with ffigen use (adjust paths to yours):
 ```bash
 dart run ffigen --compiler-opts "-I/path/to/libgit2dart/libgit2/headers/ -I/lib64/clang/12.0.1/include"
 ```
+
+## Running Tests
+
+To run all tests and generate coverage report use the following commands:
+
+```sh
+$ dart pub global activate coverage
+$ dart test --coverage="coverage"
+$ format_coverage --lcov --in=coverage --out=coverage/lcov.info --packages=.packages --report-on=lib
+```
+
+To view the generated coverage report you can use [lcov](https://github.com/linux-test-project/lcov):
+
+```sh
+$ genhtml coverage/lcov.info -o coverage/
+$ open coverage/index.html
+```