[isHeSatoshi]

Fixes #1672.

The issue reports two problems:

1. "External Dependencies" in contributing.md does not suggest that it means "things we have embedded in our repository but should be treated as external dependencies".
2. The page it links to does not answer "where/how should changes be contributed". The answer is there, but in a paragraph that comes after the list of tools, phrased as "However, if possible, enhancements, bug fixes, etc. specific to these tools should be filed against the tools directly in their respective upstream repositories."

This addresses both:

• the pointer now names what it is about and lists a few of the tools, so the scope is obvious from the contents page;
• external-repos.md gets a ## Where should a change go? section near the top, before the mechanisms are described, stating the three cases.

The three cases are the ones the page already states further down, moved up and stated plainly rather than invented:

• an enhancement or bug fix for a tool goes to the tool's own repository;
• a change that needs both a compiler change and a tool change goes in one rust-lang/rust pull request;
• a fix for a tool that your compiler change breaks goes in that same pull request.

The rest of the chapter is about the sync mechanisms, so the new section says so and gets out of the way.

No new links are added.

[rustbot]

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