Document API and its usage

I added basic usage of the CLI and plugins; I don’t to give the impression the API is the main way to test rules.

README.md

@@ -9,16 +9,143 @@ Completely and thoroughly test your Firebase security rules without connecting t
 ## Usage
 
 All you need to do is supply the security rules and some mock data, then write tests describing the expected behavior of the rules. Targaryen will interpret the rules and run the tests.
-
-Targaryen can be used in one of three ways: as a standalone command-line utility, as a set of custom matchers for [Jasmine](https://jasmine.github.io), or as a plugin for [Chai](http://chaijs.com). When a test fails, you get detailed debug information that explains why the read/write operation succeeded/failed.
 
+```js
+const assert = require('assert');
+const targaryen = require('targaryen');
+
+const rules = {
+  rules: {
+    foo: {
+      '.write': 'true'
+    }
+  }
+};
+const data = {foo: 1};
+const auth = {uid: 'someuid'};
+
+const database = targaryen.database(rules, data);
+const {allowed, newDatabase, info} = database.as(auth).write('/foo', 2);
+
+console.log('Rule evaluations:\n', info);
+assert.ok(allowed);
+
+assert.equal(newDatabase.rules, database.rules);
+assert.equal(newDatabase.root.foo.$value(), 2);
+assert.equal(newDatabase.auth, auth);
+```
+
+Targaryen three convenient ways to run tests:
+
+- as a standalone command-line utility:
+
+    ```bash
+    targaryen path/to/rules.json path/to/tests.json
+    ```
+
+- as a set of custom matchers for [Jasmine](https://jasmine.github.io):
+
+    ```js
+    const targaryen = require('targaryen/plugins/jasmine');;
+
+    describe('my security rules', function() {
+
+      beforeEach(function() {
+        jasmine.addMatchers(targaryen.matchers);
+        targaryen.setFirebaseData(require(DATA_PATH));
+        targaryen.setFirebaseRules(require(RULES_PATH));
+      });
+
+      it('should allow authenticated used to rule all data', function() {
+        expect({uid: 'foo'}).canRead('/');
+        expect(null).cannotRead('/');
+      })
+
+    });
+    ```
+
+- or as a plugin for [Chai](http://chaijs.com).
+
+    ```js
+    const chai = require('chai');
+    const targaryen = require('targaryen/plugins/chai');
+    const expect = chai.expect;
+
+    chai.use(targaryen);
+
+    describe('my security rules', function() {
+
+      before(function() {
+        targaryen.setFirebaseData(require(DATA_PATH)));
+        targaryen.setFirebaseRules(require(RULES_PATH));
+      });
+
+      it('should allow authenticated used to rule all data', function() {
+        expect({uid: 'foo'}).can.read.path('/');
+        expect(null).cannot.read.path('/');
+      })
+
+    });
+    ```
+
+When a test fails, you get detailed debug information that explains why the read/write operation succeeded/failed.
 
 See [USAGE.md](https://github.com/goldibex/targaryen/blob/master/USAGE.md) for more information.
 
+
 ## How does Targaryen work?
 
 Targaryen statically analyzes your security rules using [esprima](http://esprima.org). It then conducts two passes over the abstract syntax tree. The first pass, during the parsing process, checks the types of variables and the syntax of the rules for correctness. The second pass, during the testing process, evaluates the expressions in the security rules given a set of state variables (the RuleDataSnapshots, auth data, the present time, and any wildchildren).
 
+
+## API
+
+### `targaryen.database(rules: object|Ruleset, data: object|DataNode, now: null|number): Database`
+
+Creates a set of rules and initial data to simulate read, write and update of operations.
+
+The Database objects are immutable; to get an updated Database object with different the user auth data, rules, data or timestamp, use its `with(options)` method.
+
+
+### `Database.prototype.with({rules: {rules: object}, data: any, auth: null|object, now: number}): Database`
+
+Extends the database object with new rules, data, auth data, or time stamp.
+
+
+### `Database.prototype.as(auth: null|object): Database`
+
+Extends the database object with auth data.
+
+
+### `Database.prototype.read(path: string, now: null|number): Result`
+
+Simulates a read operation.
+
+
+### `Database.prototype.write(path: string, value: any, priority: any, now: null|number): Result`
+
+Simulates a write operation.
+
+
+### `Database.prototype.update(path: string, patch: object, now: null|number): Result`
+
+Simulates an update operation (including multi-location update).
+
+
+### `Result: {allowed: boolean, info: string, database: Database, newDatabase: Database, newValue: any}`
+
+It holds:
+
+- `allowed`: the success status;
+- `info`: the rule evaluation info;
+- `database`: the original database.
+
+For write and update operations, it also includes:
+
+- `newDatabase`: the resulting database;
+- `newValue`: the value written to the database.
+
+
 ## Why is it named Targaryen?
 
 > There were trials. Of a sort. Lord Rickard demanded trial by combat, and the