docs: add basic example to README

README.md

@@ -1,3 +1,60 @@
 # rearch-rs
 
-Coming soon!
+The README is a large WIP, but below are some basics:
+
+## Getting Started
+First, you need to define some "capsules."
+There are several ways to define capsules, depending on your toolchain and opinions toward macros.
+
+### Macros (Optional), Stable and Nightly Rust
+Note: This particular example uses the nightly-only `better-api` feature.
+You can also write macros using the stable Rust syntax further below.
+```rust
+#[capsule]
+fn count(register: SideEffectRegistrar) -> (u8, impl Fn(u8) + Clone + Send + Sync) {
+    let (state, set_state) = register(side_effects::state(0));
+    (*state, set_state)
+}
+
+#[capsule]
+fn count_plus_one() -> u8 {
+  $count + 1
+}
+
+let container = Container::new();
+let ((count, set_count), count_plus_one) = container.read((count, count_plus_one));
+```
+
+### Vanilla, Nightly Rust (`better-api` feature)
+```rust
+fn count(
+    _: CapsuleReader, register: SideEffectRegistrar,
+) -> (u8, impl Fn(u8) + Clone + Send + Sync) {
+    let (state, set_state) = register(side_effects::state(0));
+    (*state, set_state)
+}
+
+fn count_plus_one(mut get: CapsuleReader, _: SideEffectRegistrar) -> u8 {
+  get(count) + 1
+}
+
+let container = Container::new();
+let ((count, set_count), count_plus_one) = container.read((count, count_plus_one));
+```
+
+### Vanilla, Stable Rust
+```rust
+fn count(
+    _: CapsuleReader, registrar: SideEffectRegistrar,
+) -> (u8, impl Fn(u8) + Clone + Send + Sync) {
+    let (state, set_state) = registrar.register(side_effects::state(0));
+    (*state, set_state)
+}
+
+fn count_plus_one(mut reader: CapsuleReader, _: SideEffectRegistrar) -> u8 {
+  reader.read(count) + 1
+}
+
+let container = Container::new();
+let ((count, set_count), count_plus_one) = container.read((count, count_plus_one));
+```