Explain repo purpose in README

See #96

Author: samtstern

README.md

@@ -3,6 +3,18 @@
 This repository holds code snippets used in Web documentation
 on [firebase.google.com](https://firebase.google.com/docs/).
 
+These snippets are part of our documentation and best read in the context of a documentation page rather than used directly. If you're looking to get started with the Firebase Web SDK the best place to start is [quicstart-web](https://github.com/firebase/quickstart-web).
+
+## Example
+
+Consider this page:
+https://firebase.google.com/docs/database/web/lists-of-data
+
+Each snippet in the page is dynamically included from the source in this repository, in this case mostly from this file:
+https://github.com/firebase/snippets-web/blob/master/database/lists-of-data.js
+
+Each snippet has a "region tag" which is defined by `// [START tag]` and `// [END tag]` comments. The code between the tags can be included in our documentation. Keeping the code on GitHub, rather than hard-coded into the HTML of our documentation, allows us to ensure the code is correct and up to date.
+
 ## Contributing
 
 We love contributions! See [CONTRIBUTING.md](./CONTRIBUTING.md) for guidelines.