message: fix(techdocs-core): update installation instructions
diff: This is the base [Mkdocs](https://mkdocs.org) plugin used when using Mkdocs with Spotify's TechDocs. It is written in Python and packages all of our Mkdocs defaults, such as theming, plugins, etc in a single plugin.
  +[Python Package](https://pypi.org/project/mkdocs-techdocs-core/)
  +
  ## Usage
  -**Installation instructions TBD.** We haven't published it to a Python registry yet.
  +```bash
  +$ pip install mkdocs-techdocs-core
  +```
  Once you have installed the `mkdocs-techdocs-core` plugin, you'll need to add it to your `mkdocs.yml`.
  