Update README.md

Author: ericelliott

README.md

@@ -9,6 +9,15 @@ An educational student app project originally designed for [Learn JavaScript wit
 Create a repository and implement your own Checkin app. Feel free to recruit other people and work in groups (ideally up to 7 people).
 
 
+## Things to Keep in Mind
+
+* Don't export any classes. If you need to instantiate anything, use a factory function.
+* Use **pure functions** wherever you can. A pure function does not mutate anything outside itself. A pure function does not produce side effects. Given the same inputs, a pure function will always return the same output.
+* Notice how much of the program state can be represented as lists of things:
+  - A list of users
+  - A list of teams
+  - A list of checkins.
+
 ## What's a Scrum Checkin?
 
 In agile development, we strive to implement the minimum effective amount of process to enable [high velocity development](https://medium.com/javascript-scene/how-to-build-a-high-velocity-development-team-4b2360d34021).