Add instruction for using grcov to help improve unit test coverage

.gitignore

@@ -20,3 +20,5 @@ script.py
 build/
 dist/
 
+# grcov
+*.profraw

GRCOV.md

@@ -0,0 +1,24 @@
+# Getting test coverage report with grcov #
+
+Get [grcov](https://github.com/mozilla/grcov ) and install their requirements for your system.
+
+To get a complete test report run the following commands with these env
+variables set. (I found some flags they suggested broke with some of our packages.)
+
+`.bashrc`
+```shell
+export LLVM_PROFILE_FILE="your_name-%p-%m.profraw"
+export RUSTDOCFLAGS="-Cpanic=abort"
+```
+
+1. `RUSTFLAGS="-Zinstrument-coverage -Ccodegen-units=1 -Copt-level=0 -Coverflow-checks=off -Zpanic_abort_tests" CARGO_INCREMENTAL=0 RUSTC_BOOTSTRAP=1 cargo build`
+1. `RUSTFLAGS="-Zinstrument-coverage -Ccodegen-units=1 -Copt-level=0 -Coverflow-checks=off -Zpanic_abort_tests" CARGO_INCREMENTAL=0 RUSTC_BOOTSTRAP=1 cargo test`
+1. `grcov . -s . --binary-path ./target/debug/ -t html --branch --ignore-not-existing -o ./target/debug/coverage/`
+
+
+## Known issues ##
+
+- If you run `grcov` on Windows you will need to run it as admin or it will fail
+with an error related to symlinks. [Issue 561](https://github.com/mozilla/grcov/issues/561)
+- `[ERROR] Execution count overflow detected.` can occur when running `grcov`
+  but is not fatal and a report will still be created. [Issue 613](https://github.com/mozilla/grcov/issues/613)