Add documentation for k4run custom arguments (#169)

doc/k4run-args.md

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+# Adding custom arguments to `k4run`
+
+It is possible to extend `k4run` with custom arguments from a steering file using `k4FWCore.parseArgs`.
+
+Example:
+
+```python
+from k4FWCore.parseArgs import parser
+parser.add_argument("--trackingOnly", action="store_true", help="Run only track reconstruction", default=False)
+my_opts = parser.parse_known_args()[0]
+
+# later
+if my_opts.trackingOnly:
+    # only run track reconstruction
+```
+
+Behind the scenes parser is just a normal instance of pythons [`argparse.ArgumentParser`](https://docs.python.org/3/library/argparse.html), please refer to its documentation for usage details. The only important thing to keep in mind is to always use `parse_known_args()` instead of `parse_args()` so that the normal `k4run` arguments keep working.