docs: add coverage support

Signed-off-by: Alex Chi <[email protected]>

site/src/SUMMARY.md

@@ -17,6 +17,7 @@
   - [Target runners](book/target-runners.md)
   - [Other options](book/other-options.md)
   - [Environment variables](book/env-vars.md)
+  - [Coverage](book/coverage.md)
 - [Machine-readable output](book/machine-readable.md)
 - [Configuration](book/configuration.md)
 - [JUnit support](book/junit.md)

site/src/book/coverage.md

@@ -0,0 +1,18 @@
+# Coverage
+
+[cargo-llvm-cov](https://github.com/taiki-e/cargo-llvm-cov) has support for nextest.
+Simply run the following command to generate coverage report with nextest:
+
+```
+cargo install cargo-llvm-cov
+cargo llvm-cov nextest
+```
+
+## GitHub Action
+
+You may install cargo-llvm-cov and nextest on GitHub Action, and then run llvm-cov with nextest.
+
+```yaml
+- uses: taiki-e/install-action@cargo-llvm-cov
+- uses: taiki-e/install-action@nextest
+```