exa: update README

Signed-off-by: Matt Liberty <[email protected]>

Author: maliberty

src/exa/README.md

@@ -1 +1,20 @@
-Example
+# Example
+
+This is an example tool to demonstrate how to setup a new tool in OpenROAD.
+
+## Commands
+
+### Example Instance
+
+This command makes an instance.
+
+```tcl
+example_instance
+    [-name name]
+```
+
+#### Options
+
+| Switch Name | Description |
+| ----- | ----- |
+| `[-name]` | The name of the instance to create`. Defauts to example. |