Auto merge of #12514 - changm:issue12396, r=SimonSapin

Issue 12396 to add developer documentation to readme.

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Just some added developer documentation that was useful for me while starting to develop servo.

This adds the documentation about running ./mach clean with a partially built servo but before linking openssl.

It also adds the -Z flag under command line flags for debugging help.
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- [X ] `./mach build -d` does not report any errors
- [ X] `./mach test-tidy` does not report any errors
- [ X] These changes fix #12396  (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it is a documentation update only.

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