Add 'Running'

Running.md

@@ -0,0 +1,17 @@
+Use `cargo` to run an input file.
+
+```bash
+git clone https://tildegit.org/karx/sandwich
+cd sandwich
+cargo run <filename>
+```
+
+Alternatively, you can install `sandwich` to your cargo bin directory.
+
+```bash
+git clone https://tildegit.org/karx/sandwich
+cd sandwich
+cargo install --path /path/to/sandwich/root
+# Now, to run,
+sandwich <filename>
+```