docs(style): Specify the frontmatter style

Taken from [a style team
discussion](rust-lang/style-team#212 (comment)).

Assumptions on my part:
- I don't need to specify the lack of trailing spaces after the code
  fence / infostring
- We aren't specifying when to include the infostring (one idea being if
  there is no shebang)
- Keep it simple and have a single example instead of showing allowed several
  variations

Author: epage

src/doc/style-guide/src/nightly.md

@@ -5,3 +5,27 @@ This chapter documents style and formatting for nightly-only syntax. The rest of
 Style and formatting for nightly-only syntax should be removed from this chapter and integrated into the appropriate sections of the style guide at the time of stabilization.
 
 There is no guarantee of the stability of this chapter in contrast to the rest of the style guide. Refer to the style team policy for nightly formatting procedure regarding breaking changes to this chapter.
+
+### Frontmatter
+
+*Location: Placed before comments and attributes in the [root](index.html).*
+
+*Tracking issue: [#136889](https://github.com/rust-lang/rust/issues/136889)*
+
+*Feature gate: `frontmatter`*
+
+There should be no blank lines between the frontmatter and either the start of the file or a shebang.
+There can be zero or one line between the frontmatter and any following content.
+
+The frontmatter fence should use the minimum number of dashes necessary for the contained content.
+If an infostring is present after the opening fence, there should be one space separating them.
+
+```rust
+#!/usr/bin/env cargo
+--- cargo
+[dependencies]
+regex = "1"
+---
+
+fn main() {}
+```