Downgrade nightly for now to workaround pyo3 issue

PyO3/pyo3#144

Author: msanders

README.md

@@ -24,13 +24,24 @@ Currently supported on macOS, Windows, and X11 with the XTest extension.
 
 ### Installation
 
-	$ git clone git://github.com/autopilot-rs/autopy-rs.git
-	$ cd autopy
-    $ rustup install nightly
-    $ rustup default nightly
-  	$ pip install -U setuptools-rust
-	$ python setup.py build
-	# python setup.py install
+First, see if a binary wheel is available for your machine by running:
+
+    $ pip install -U autopy
+
+If that fails, install [rustup](https://rustup.rs) and then run:
+
+    $ rustup default nightly-2018-04-19
+    $ pip install -U setuptools-rust
+    $ pip install -U autopy
+
+Another option is to compile from the latest source on the GitHub repository:
+
+    $ git clone git://github.com/autopilot-rs/autopy-rs.git
+    $ cd autopy
+    $ rustup default nightly-2018-04-19
+    $ pip install -U setuptools-rust
+    $ python setup.py build
+    # python setup.py install
 
 ### Hello World