docs: add usage example

README.md

@@ -22,18 +22,68 @@ To use batch insertions you must enable `batch_insert` feature:
 pmtree = { git = "https://github.com/Rate-Limiting-Nullifier/pmtree", features = ["batch_insert"] }
 ```
 
-## Clone & Build
-```bash
-git clone git@github.com:Rate-Limiting-Nullifier/pmtree.git && cd pmtree
-cargo build --release
-```
+## Example
 
-## Run tests
-```bash
-cargo test --release
-```
+In-Memory DB (HashMap) + Keccak
+```rust
+struct MemoryDB(HashMap<DBKey, Value>);
+struct MyKeccak(Keccak);
 
-## Docs
-```bash
-cargo docs --open
+impl Database for MemoryDB {
+    fn new(_dbpath: &str) -> Result<Self> {
+        Ok(MemoryDB(HashMap::new()))
+    }
+
+    fn load(_dbpath: &str) -> Result<Self> {
+        Err(Error("Cannot load in-memory DB".to_string()))
+    }
+
+    fn get(&self, key: DBKey) -> Result<Option<Value>> {
+        Ok(self.0.get(&key).cloned())
+    }
+
+    fn put(&mut self, key: DBKey, value: Value) -> Result<()> {
+        self.0.insert(key, value);
+
+        Ok(())
+    }
+}
+
+impl Hasher for MyKeccak {
+    type Fr = [u8; 32];
+
+    fn default_leaf() -> Self::Fr {
+        [0; 32]
+    }
+
+    fn serialize(value: Self::Fr) -> Value {
+        value.to_vec()
+    }
+
+    fn deserialize(value: Value) -> Self::Fr {
+        value.try_into().unwrap()
+    }
+
+    fn hash(input: &[Self::Fr]) -> Self::Fr {
+        let mut output = [0; 32];
+        let mut hasher = Keccak::v256();
+        for element in input {
+            hasher.update(element);
+        }
+        hasher.finalize(&mut output);
+        output
+    }
+}
+
+fn main() {
+    let mut mt = MerkleTree::<MemoryDB, MyKeccak>::new(2, "abacaba").unwrap();
+
+    assert_eq!(mt.capacity(), 4);
+    assert_eq!(mt.depth(), 2);
+
+    mt.update_next(hex!(
+        "c1ba1812ff680ce84c1d5b4f1087eeb08147a4d510f3496b2849df3a73f5af95"
+    ))
+    .unwrap();
+}
 ```