Change subsections to be hierarchical headings in README (#574)

Author: csarven

README.md

@@ -3,14 +3,14 @@
 A set of design principles to be used when designing web platform technologies. These principles have been collected by the [Technical Architecture Group](https://www.w3.org/2001/tag/) and developed along with the [Ethical Web principles](https://github.com/w3ctag/ethical-web-principles/).
 
 
-# Building
+## Building
 
 You will need [GNU Make](https://www.gnu.org/software/make/) and Python 3.
 
 With those dependencies, simply invoke `make` to build the document.
 
 
-# Contributing
+## Contributing
 
 Contributions are welcome to anyone who follows [our code of conduct](https://www.w3.org/Consortium/cepc/).