Add more docs (#2)

Author: yugui

README.md

@@ -39,6 +39,124 @@ Just `git clone`.
 $ git clone https://github.com/yugui/jsonnetunit.git
 ```
 
+## Getting started
+### How to write tests
+1. Create a test file
+
+   Test files must be `.jsonnet` files which manifestize a result of `test.suite` function.
+   
+   ```jsonnet
+   local test = import "path/to/jsonnetunit/test.jsonnet";
+   test.suite({
+   })
+   ```
+2. Add test cases
+
+    `test.suite` function takes an object which contains fields prefixed with `test`.
+    You can add arbitrary number of such fields.  `test.suite` does not directly use any other fields.
+    
+    Individual test fields must have at least two fields:
+    * `actual` field: There must be a field named `actual`. This is the actual value to be verified. 
+    * expectation field: There must be another field which describes an expectation.  This expectation is used to verify the `actual` value.
+      
+      ```jsonnet
+      test.suite({
+          testFoo: {
+              actual: std.length('foo'),
+              expect: 3,
+          },
+      })
+      ```
+      
+      
+      The interpretation of the expectation depends on the name of the expectation field. The name `expect` in the example means that it expects that `actual` field is equal to the given value.
+
+### Simple expectations
+
+Expectation Field Name | Description               | Example
+-----------------------|---------------------------|---------------------
+`expect`               | value equality            | `{actual: 1+1, expect: 2}`
+`expectNot`            | value inequality          | `{actual: 1+1, expect: 3}`
+`expectLt`             | less than                 | `{actual: 1+1, expect: 3}`
+`expectLe`             | less than or equal to     | `{actual: 1+1, expect: 3}`
+`expectGt`             | greater than              | `{actual: 1+1, expect: 1}`
+`expectGe`             | greater than or equal to  | `{actual: 1+1, expect: 1}`
+
+### expectThat
+You can describe an abitrary expectation with `expectThat`.
+This expectation field takes a unary function or an object.
+
+The function must take an actual value and it must return if the value satisfies your expectation in a boolean value.
+
+```jsonnet
+{
+  actual: ultimateAnswerToLifeTheUniverseAndEverything(),
+  expectThat: function(x) x == 42,
+}
+```
+
+When you pass an object, the object must have two fields `actual` and `result`.
+The first field `actual` is overridden with the `actual` value of the test case on evaluation.
+The second field `result` must be an boolean which describes whether `actual` field satisfies your expectation.
+In this case, you can optionally specifies a custom `description` of the expectation.  This is used as a part of error message when the test case fails.
+
+```jsonnet
+{
+  actual: ultimateAnswerToLifeTheUniverseAndEverything(),
+  expectThat: {
+    actual: error "to be overridden",
+    result: self.actual == 42,
+    description: "Expect %d to be equal to the known value" % self.actual,
+  },
+}
+```
+
+### Custom expectation matcher
+You can also define your own expectation matcher.
+
+1. Define a binary function which takes `actual` and `expected` values. This function must return an object
+   derived from `matcher.jsonnet` and must have the following three fields.
+   * `satisfied`: (boolean) Returns if `self.actual` satisfies your expectation
+   * `positiveMessage`: (string) Returns an error message to be returned when `self.actual` does not satisfies your expectation.
+   * `negativeMessage`: (string) Returns an error message to be returned when `self.actual` does not satisfies the negation of the expectation.
+
+   e.g.
+   ```jsonnet
+   local setMatcher(actual, expected) = import "path/to/matcher.jsonnet" {
+       satisfied: std.set(actual) == std.set(expected),
+       positiveMessage: "Expected " + actual + " to be equal to " + expected + " as a set",
+       negativeMessage: "Expected " + actual + " not to be equal to " + expected + " as a set",
+   };
+   ```
+2. Define your expectation field name in the `matchers` field of the test suite.
+   
+   e.g.
+   ```jsonnet
+   test.suite({
+       testEq: {
+           actual: [6, 7, 2, 3, 7],
+           expectSetEq: [2, 3, 6, 7],
+       },
+       testNe: {
+           actual: [6, 7, 2, 3, 7],
+           expectSetNe: [1, 2, 3, 4, 5],
+        }
+   }) {
+       matchers+: {
+           // Define a new expectation field name "expectSetEq" for set equality
+           expectSetEq: {
+               matcher: setMatcher,
+               expectationType: true,
+           },
+            // Define a new expectation field name "expectSetNe" for set inequiality
+           expectSetNe: {
+               matcher: setMatcher,
+               expectationType: false,
+           },
+       },
+   }
+   ```
+
 # Copyright
 Copyright 2016 Yuki Yugui Sonoda