added brief readme for directed test flow

Author: Saad Khalid

dv/uvm/core_ibex/directed_tests/README.md

@@ -0,0 +1,18 @@
+# Directed Tests
+
+This directory contains the custom directed tests as well as scripts and headers for running directed tests vendored from various open source repositories.
+
+Currently following open source test suites are vendored:
+- [riscv-tests](https://github.com/riscv-software-src/riscv-tests)
+- [riscv-arch-tests](https://github.com/riscv-non-isa/riscv-arch-test)
+- epmp-tests ([fork](https://github.com/Saad525/riscv-isa-sim) from an opensource [repo](https://github.com/joxie/riscv-isa-sim))
+
+## Generating test list
+
+To generate a testlist containing all of the directed tests (custom + tests from vendored repos).
+
+```
+python3 gen_testlist.py --add_tests riscv-tests,riscv-arch-tests,epmp-tests
+```
+
+Please note that the custom directed tests needs to be added in the `gen_testlist.py` script and it needs to be run in order to update `directed_testlist.yaml`.