Add fp warning

week1/03-homework1.md

@@ -14,7 +14,19 @@ You can always refer to:
   * [this guide](https://gitlab.epfl.ch/lamp/cs-210-functional-programming-2019/blob/master/week1/02-grading-and-submission.md) for details on the submission system.
     **Make sure to submit your assignment before the deadline written in [README.md](/README.md)**
   * [The documentation of the Scala standard library](https://www.scala-lang.org/files/archive/api/2.13.1)
-  * [The documentation of the Java standard library](https://docs.oracle.com/en/java/javase/11/docs/api/index.html)
+  * [The documentation of the Java standard
+    library](https://docs.oracle.com/en/java/javase/11/docs/api/index.html)
+
+
+# Be functional!
+
+This course is about **functional** programming, where you'll learn to program
+without using mutation, therefore you're not allowed to use the following
+constructs in the homeworks:
+- `var`
+- `while`
+- `return`
+- Any class in the `scala.collection.mutable` package
 
 # Exercise 1: Pascal's Triangle