book: Add some more placeholder sections to the user guide

book/src/SUMMARY.md

@@ -3,6 +3,8 @@
 [halo2](README.md)
 - [Concepts](concepts.md)
 - [User Documentation](user.md)
+  - [A simple example](user/simple-example.md)
+  - [Lookup tables](user/lookup-tables.md)
   - [Gadgets](user/gadgets.md)
   - [Tips and tricks](user/tips-and-tricks.md)
 - [Design](design.md)

book/src/user.md

@@ -1 +1,5 @@
 # User Documentation
+
+You're probably here because you want to write circuits? Excellent!
+
+This section will guide you through the process of creating circuits with halo2.

book/src/user/lookup-tables.md

@@ -0,0 +1,7 @@
+# Lookup tables
+
+In normal programs, you can trade memory for CPU to improve performance, by pre-computing
+and storing lookup tables for some part of the computation. We can do the same thing in
+halo2 circuits!
+
+TODO

book/src/user/simple-example.md

@@ -0,0 +1,19 @@
+# A simple example
+
+Let's start with a simple circuit, to introduce you to the common APIs and how they are
+used. The circuit will take a public input $c$, and will prove knowledge of two private
+inputs $a$ and $b$ such that
+
+$$a * b = c.$$
+
+```rust
+# extern crate halo2;
+use halo2::arithmetic::FieldExt;
+
+struct MyCircuit<F: FieldExt> {
+    a: F,
+    b: F,
+}
+```
+
+TODO