What's the problem this feature will solve?
I want to set internal links/anchors in my README.md/long_description in markdown format (for example See [the documentation](#documentation) for more information which links to a ## Documentation heading or an explicit <a name="documentation"></a> anchor). Ideally I'd like to use the same internally linked Markdown source for both GitHub and PyPI.

Describe the solution you'd like
Explicit link creation via <a name> and <a id> should be supported (maybe only <a name>, depending on the potential to disturb any Javascript code).

Ideally GitHub-compatible automatic anchor generation for headings should also be supported.

Additional context
A GitHub README.md supports two ways of setting anchors:

• Implicitly, by generating an anchor from a heading (an <a id="lower-case-with-some-chars-replaced-by-hyphens">)
• Explicitly, by placing <a id="..."> or <a name="..."> tags.

Warehouse does not automatically generate anchors for headings. It also strips id and name attributes from HTML embedded in the Markdown file. This leaves no possibility of linking to another location within the description (without using an absolute link to PyPI.org, which is undesirable since the README is typically used in other places like GitHub too).