Update README with invocation script usage

README.md

@@ -1,3 +1,19 @@
 # rusttest-to-dg
 
 Converts `rustc` testcases into dejagnu testcases for `gccrs`
+
+# Usage
+
+This tool was invoked by the script [run](./run.sh). Furthermore, we need the rustc and gccrs source code to be downloaded on the system. And add their paths to the environment variables `GCCRS_PATH`, `RUST_PATH` and `RUSTTEST_TO_DG_PATH`
+
+```bash
+export GCCRS_PATH=/path/to/gccrs
+export RUST_PATH=/path/to/rust
+export RUSTTEST_TO_DG_PATH=/path/to/rusttest-to-dg
+```
+
+Then, simply run the script [run](./run.sh)
+
+```bash
+bash run.sh
+```