Auto merge of #20753 - pyfisch:patch-1, r=emilio

Build note for Ubuntu 16.04

See #20712

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/20753)
<!-- Reviewable:end -->

README.md

@@ -88,6 +88,8 @@ sudo apt install git curl autoconf libx11-dev \
 
 If you using a version prior to **Ubuntu 17.04** or **Debian Sid**, replace `libssl1.0-dev` with `libssl-dev`.
 
+If you are using **Ubuntu 16.04** run `export HARFBUZZ_SYS_NO_PKG_CONFIG=1` before building to avoid an error with harfbuzz.
+
 If you are on **Ubuntu 14.04** and encountered errors on installing these dependencies involving `libcheese`, see [#6158](https://github.com/servo/servo/issues/6158) for a workaround.
 
 If `virtualenv` does not exist, try `python-virtualenv`.