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.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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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README.md

@@ -34,6 +34,8 @@ brew install openssl
 brew link --force openssl
 ```
 
+If you've already partially compiled servo but forgot to do this step, run ./mach clean, link openssl, and recompile.
+
 On Debian-based Linuxes:
 
 ``` sh
@@ -176,6 +178,7 @@ URL with servo).
   `INTERVAL` seconds
 - `-s SIZE` sets the tile size for painting; defaults to 512
 - `-z` disables all graphical output; useful for running JS / layout tests
+- `-Z help` displays useful output to debug servo
 
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