updated readme with usage instructions

README.md

@@ -2,8 +2,8 @@
 [![Downloads](https://pepy.tech/badge/keithleygui)](https://pepy.tech/project/keithleygui)
 [![Build Status](https://travis-ci.com/OE-FET/keithleygui.svg?branch=master)](https://travis-ci.com/OE-FET/keithleygui)
 
-**Warning:** Keithleygui v1.1.4 will be the last release that is developed and tested for
-Python 2.7. Future releases will only support Python 3.6 and higher.
+**Warning:** Keithleygui v1.1.4 is the last release that is developed and tested
+for Python 2.7. Newer releases require Python 3.6 or higher.
 
 # keithleygui
 A high-level user interface for Keithley 2600 series instruments which allows
@@ -13,6 +13,9 @@ of readings from the Keithley, the data from an IV-curve is buffered locally on
 the instrument and only transferred to CustomXepr after completion of a
 measurement.
 
+`keithleygui` build on the Python driver provided by
+[keithley2600](https://github.com/OE-FET/keithley2600).
+
 ![Screenshot of the user interface](screenshots/KeithleyGUI.png)
 
 ## Installation
@@ -25,6 +28,40 @@ Or install the latest development version from GitHub:
 $ pip install git+https://github.com/OE-FET/keithleygui
 ```
 
+## Usage
+In the terminal, run `keithleygui` to start the user interface. If your provide the
+`--verbose` option, log output showing the communication with the Keithley will be
+printed to the console.
+
+To start the user interface in a running Python terminal:
+
+```Python
+from PyQt5 import QtWidgets
+from keithleygui import KeithleyGuiApp
+
+app = QtWidgets.QApplication([])
+
+keithley_gui = KeithleyGuiApp()
+keithley_gui.show()
+app.exec()
+```
+
+You can optionally provide an existing `Keithley2600` instance to be used by the GUI:
+
+```Python
+from PyQt5 import QtWidgets
+from keithley2600 import Keithley2600
+from keithleygui import KeithleyGuiApp
+
+app = QtWidgets.QApplication([])
+
+keithley = Keithley2600('visa_address')
+keithley_gui = KeithleyGuiApp(keithley)
+keithley_gui.show()
+app.exec()
+```
+
+
 ## System requirements
 
 - Linux or macOS