chore: add README.md

README.md

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+# Coverage
+
+Coverage is a simple tool that checks per package coverage and fails if is not met.
+
+```bash
+go get github.com/gkampitakis/coverage
+```
+
+## Usage
+
+You can use it by calling in your `TestMain`. For more information around [TestMain](https://pkg.go.dev/testing#hdr-Main).
+
+
+```go
+func TestMain(m *testing.M) {
+	coverage.Run(m, 95)
+}
+```
+
+What `coverage.Run` does, it runes the tests with `t.Run()` and then depending on the coverage fails the tests and calls `os.Exit` with correct exit code.
+
+In case you have "clean" up code after running your tests `coverage.Run` support passing a callback function `func(t *testing.M)` for running your code. 
+
+e.g. 
+
+```go
+func TestMain(m *testing.M) {
+	coverage.Run(m, 91, func(t *testing.M) {
+		fmt.Println("tests are done")
+	})
+}
+```
+
+## Example
+
+```
+PASS
+coverage: 89.5% of statements
+
+FAIL    Coverage threshold not met 91.0 >= 85.0 for my-package/example
+
+FAIL    my-package/example        0.185s
+```
+
+> `coverage: 89.5% of statements` reports slightly different number from the
+`testing.Coverage`. Couldn't find a way to fix this or an explanation.
+
+`coverage.Run` will also fail when tests are run with no `cover` flag.
+
+e.g.
+
+```
+PASS
+
+FAIL    coverage is not enabled. You can enable by using `-cover`
+```