Add the content of the package.

README.md

+The `boolean_selector` package defines a simple and flexible syntax for boolean
+expressions. It can be used for filtering based on user-defined expressions. For
+example, the [`test`][test] package uses boolean selectors to allow users to
+define what platforms their tests support.
+
+[test]: http://github.com/dart-lang/test
+
+The boolean selector syntax is based on a simplified version of Dart's
+expression syntax. Selectors can contain identifiers, parentheses, and boolean
+operators, including `||`, `&&`, `!`, and `? :`. Any valid Dart identifier is
+allowed, and identifiers may also contain hyphens. For example, `chrome`,
+`chrome || content-shell`, and `js || (vm && linux)` are all valid boolean
+selectors.
+
+A boolean selector is parsed from a string using
+[`new BooleanSelector.parse()`][parse], and evaluated against a set of variables
+using [`BooleanSelector.evaluate()`][evaluate]. The variables may be supplied as
+a list of strings, or as a function that takes a variable name and returns its
+value. For example:
+
+[parse]: https://www.dartdocs.org/documentation/boolean_selector/latest/boolean_selector/BooleanSelector/BooleanSelector.parse.html
+
+[evaluate]: https://www.dartdocs.org/documentation/boolean_selector/latest/boolean_selector/BooleanSelector/evaluate.html
+
+```dart
+import 'package:boolean_selector/boolean_selector.dart';
+
+void main(List<String> args) {
+  var selector = new BooleanSelector.parse("(x && y) || z");
+  print(selector.evaluate((variable) => args.contains(variable)));
+}
+```
+
+## Versioning
+
+If this package adds new features to the boolean selector syntax, it will
+increment its major version number. This ensures that packages that expose the
+syntax to their users will be able to update their own minor versions, so their
+users can indicate that they rely on the new syntax.