Add instructions for file association support

[thejcannon]

This was the simplest solution, as it's just instructions for the user. Tested it out on my Windows 10 machine.

Alternate solutions would be:

• Provide these in batch/powershell files for the user to run (unfortunately you can't substitute env vars in a .reg file, so that's out)
• Add new command to pyenv itself to install these (makes uninstall much nicer I assume)

Additionally, some thought can be given in the future to support shebang lines, like the py launcher does, but until then 🤷‍♂️.

[thejcannon]

Fixes #152

[thejcannon]

I'll also add, I'm reasonably sure using the logo in this manner is safe under the Trademark Usage Policy: https://www.python.org/psf/trademarks/

[bkeryan]

Using REG_EXPAND_SZ to expand the environment variable also seems to work:

reg add "HKCR\Python.Pyenv\shell\open\command" /t REG_EXPAND_SZ /d "\"^%USERPROFILE^%\.pyenv\pyenv-win\shims\python.bat\" \"%1\""

[bkeryan]

If you run the command at the command line, you need to use %1, but if you put it in a batch file, you need to use %%1 to escape the percent. Adding a caret ("^%1") doesn't seem to help.

[kirankotari]

@thejcannon @bkeryan I am not very good in reg add so I need experts like you to test them and validate. If everything is good and it doesn't break or gives any errors. I am good to merge it.

[thejcannon]

I actually no longer use Windows anymore, so I'll tap out.

[Spitfire1900]

I am very much on the side that this should be implemented as part of pyenv install itself.