doc(readme): describe gulp tasks

fixes dart-archive#212

Author: vicb

README.md

@@ -3,3 +3,15 @@
 
 ts2dart is a TypeScript to Dart transpiler. It's in its very early days and under heavy development,
 not ready for production use.
+
+## Installation
+
+- execute `npm i` to install the dependencies,
+- the Dart SDK must be available to run end to end tests.
+
+## Gulp tasks
+
+- `gulp watch` executes the unit tests in watch mode (use `gulp test.unit` for a single run),
+- `gulp test.e2e` executes the e2e tests,
+- `gulp test.check-format` checks the source code formatting using `clang-format`,
+- `gulp test` runs unit tests, e2e tests and checks the source code formatting.