Add instructions for code coverage report generation (#1963)

DEVELOPER_INSTRUCTIONS.md

@@ -0,0 +1,26 @@
+Code Coverage Report Generation
+---------------------------------
+
+Code coverage report can be generated using [grcov](https://github.com/mozilla/grcov).
+
+### Using Nightly Rust
+
+To generate [gcov-based](https://github.com/mozilla/grcov#example-how-to-generate-gcda-files-for-cc) coverage report
+
+```bash
+$ export CARGO_INCREMENTAL=0
+$ export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zpanic_abort_tests -Cpanic=abort"
+$ export RUSTDOCFLAGS="-Cpanic=abort"
+$ cargo build <options...> # e.g., --features feat_os_unix
+$ cargo test <options...> # e.g., --features feat_os_unix test_pathchk
+$ grcov . -s . --binary-path ./target/debug/ -t html --branch --ignore-not-existing --ignore build.rs --excl-br-line "^\s*((debug_)?assert(_eq|_ne)?\#\[derive\()" -o ./target/debug/coverage/
+$ # open target/debug/coverage/index.html in browser
+```
+
+if changes are not reflected in the report then run `cargo clean`  and run the above commands.
+
+### Using Stable Rust
+
+If you are using stable version of Rust that doesn't enable code coverage instrumentation by default 
+then add `-Z-Zinstrument-coverage` flag to `RUSTFLAGS` env variable specified above.
+