Create initial CITATION file

Author: GimmickNG

CITATION.cff

@@ -0,0 +1,15 @@
+cff-version: 1.2.0
+message: "Feel free to use this, if you want to cite this library."
+authors:
+- name: "Pycom Limited"
+- given-names: "Jones"
+- name: "The Micropython Modbus team"
+title: "Micropython Modbus"
+abstract: "A lightweight Modbus TCP/RTU library for Micropython"
+version: "2.3.4"
+license: "GPLv3"
+date-released: 2017-12-18
+url: "https://github.com/brainelectronics/micropython-modbus"
+keywords:
+- micropython
+- modbus