Document auto theme change in README

Is your feature request related to a problem? Please describe.

As explained in #690, it is not yet possible to switch the theme automatically on the Vercel instance. Nevertheless, there are multiple ways to get the card to switch themes on the client side. The methods for doing this (i.e. transparent background, #gh-dark-mode-only and source media) need to be explained in the README so that users are aware these options exist.

Describe the solution you’d like

It would be great if README would have a section for setting up dynamic themes. This section should explain:

• We can not infer the browser theme on the server since GitHub will re-upload the cards and serve them from their CDN (see https://github.com/anuraghazra/github-readme-stats/issues/79#issuecomment-663706819).
• That users can use the transparent theme.

![Anurag's GitHub stats](https://github-readme-stats.vercel.app/api?username=anuraghazra&show_icons=true&theme=transparent)

• That users can use bg_color=00000000 to set a transparent background.

![Anurag's GitHub stats](https://github-readme-stats.vercel.app/api?username=anuraghazra&show_icons=true&theme=dark&bg_color=00000000)

• They can use GitHub’s theme context tags to switch the theme automatically (see https://github.blog/changelog/2021-11-24-specify-theme-context-for-images-in-markdown/) based on the set GitHub theme.

[![Anurag's GitHub stats](https://github-readme-stats.vercel.app/api?username=anuraghazra&theme=dark#gh-dark-mode-only)](https://github.com/anuraghazra/github-readme-stats#gh-dark-mode-only)
[![Anurag's GitHub stats](https://github-readme-stats.vercel.app/api?username=anuraghazra&theme=default#gh-light-mode-only)](https://github.com/anuraghazra/github-readme-stats#gh-light-mode-only)

• That they can use GitHub’s new media feature in the HTML <picture> element to switch the theme automatically based on the set GitHub theme. This option, however, requires the user to refresh the page before the card theme is updated.

<a href="https://github.com/anuraghazra/github-readme-stats">
<picture>
  <source media="(prefers-color-scheme: dark)" srcset="https://github-readme-stats.vercel.app/api?username=anuraghazra&theme=dark">
  <img alt="Shows Anuraghazra's GitHub Stats." src="https://github-readme-stats.vercel.app/api?username=anuraghazra&theme=default">
</picture>
</a>

<picture> <source media="(prefers-color-scheme: dark)" srcset="https://github-readme-stats.vercel.app/api?username=anuraghazra&theme=dark"> </picture>

[lostgirljourney]

I want to get assigned to this.

[rickstaa]

Done in #2229.