README.md: add usage and examples

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>

README.md

@@ -1,2 +1,59 @@
 # node
 A parent/children data structure
+
+# Usage
+
+Add as a dependency in your `pom.xml`:
+
+    <dependency>
+        <groupId>net.kemitix</groupId>
+        <artifactId>node</artifactId>
+        <version>${node.version}</version>
+    </dependency>
+
+The library consits of an interface `Node` and an implementation `NodeItem`.
+
+## Create a root node
+
+    Node<String> root = new NodeItem<>("[root]");
+
+## Get the contents of the node
+
+    String rootData = root.getData(); // returns "[root]"
+
+## Add a child node
+
+    root.addChild("child");
+
+The tree now looks like:
+
+    "[root]"
+    \-> "child"
+
+## Get the child node
+
+    Node<String> childNode = root.getChild("child");
+
+## Create a chain of nodes
+
+    root.createDescendantLine(Arrays.asList("alpha", "beta", "gamma"));
+
+    "[root]"
+    |-> "child"
+    \-> "alpha"
+        \-> "beta"
+            \-> "gamma"
+
+## Walk the tree to find a node
+
+    Optional<Node<String>> foundNode = root.walkTree(Arrays.asList(
+        "alpha", "beta", "gamma"));
+    if (foundNode.isPresent()) {
+        String betaData = foundNode.get().getParent().getData();
+            // returns "beta"
+    }
+
+## Get all children of a node
+
+    Set<Node<String>> children = root.getChildren();
+    children.size(); // returns 2 ("child" and "alpha")