Add repology shield to README

[jayvdb]

How would this feature be useful?
I believe it is important for pipx to be available as distro packages, and ensure they are kept updated. A repology shield would help achieve that. e.g. https://github.com/lilydjwg/nvchecker has it in their README.

Currently there are two entries in repology which cover pipx:

• pipx
• python:pipx

I assume that those two can be merged.

Here is what each shield looks like. Obviously it will be more impressive once the two are merged.

Describe the solution you'd like
In markdown syntax it would be:

[![Packaging status](https://repology.org/badge/vertical-allrepos/pipx.svg)](https://repology.org/metapackage/pipx/versions)

[gaborbernat]

PR welcome.