Document upgrade failure from before 0.3.0

Author: labkey-nicka

README.md

@@ -21,6 +21,8 @@ To install, simply use `pip`:
 $ pip install labkey
 ```
 
+**Note:** For users who installed this package before it was published to PyPI (before v0.3.0) it is recommended you uninstall and reinstall the package rather than attempting to upgrade. This is due to a change in the package's versioning semantics.
+
 # Credentials
 As of v0.4.0 this API no longer supports using a ``.labkeycredentials.txt`` file, and now uses the .netrc files similar to the other labkey APIs. Additional .netrc [setup instructions](https://www.labkey.org/wiki/home/Documentation/page.view?name=netrc) can be found at the link.