Update CONTRIBUTING.md with npm and ES modules info [skip ci] (#205)

* Update CONTRIBUTING.md with npm and ES modules info [skip ci]

CONTRIBUTING.md

@@ -2,6 +2,13 @@
 
 Dear contributor, we are glad that you want to improve our product. Our products have high quality requirements, all the changes should satisfy those as well.
 
+Each Vaadin component has it's own GitHub repository, where development is done. The `vaadin-core` project is just a top-level bundle without actual production code.
+Familiarize yourself with the code and try to follow the same syntax conventions to make it easier for us to accept your pull requests.
+
+The following instructions are common for all Vaadin components.
+
+### Submitting a Pull Request
+
 To submit a PR follow these minimum requirements:
 
   - Make sure your code is compliant with our code linters: `gulp lint`
@@ -13,9 +20,10 @@ We will review the PR and make a decision on it.
 
 ### Advanced requirements
 
-Advanced requirements affect the PR handling time. The more requirements are met, the sooner PR will be merged. Otherwise, we will take care of those missing parts in case of PR approvement.
+Advanced requirements affect the PR handling time. The more requirements are met, the sooner PR will be merged.
+Otherwise, we will take care of those missing parts in case of PR approvement.
 
-  - Minimal API to fulfill the requirements of the idea (when modifying the API do not forget to to run `polymer analyze` to update analysis file)
+  - Minimal API to fulfill the requirements of the idea
 
   - 100% coverage with automated tests
     - Should pass on each supported platform (desktop/mobile):
@@ -27,7 +35,7 @@ Advanced requirements affect the PR handling time. The more requirements are met
       - iOS Safari (9, latest, any platform)
       - Chrome on Android (latest, any platform)
 
-  - Live Demo with a code example demonstarting the feature
+  - Live Demo with a code example demonstrating the feature
     - Not required for a bug fix
 
   - Documented
@@ -41,7 +49,7 @@ Advanced requirements affect the PR handling time. The more requirements are met
 
   - Visuals are themable:
     - **Supports both lumo and material themes**
-    - theming parts / state attributes are exposed
+    - themable parts / state attributes are exposed
     - documented
     - covered with visual tests (optional)
 
@@ -49,3 +57,52 @@ Advanced requirements affect the PR handling time. The more requirements are met
 
   - RTL (optional)
 
+### Converting from HTML Imports to ES modules
+
+Vaadin components use HTML Imports in the mainline codebase. In order to use a Vaadin component
+as ES module, the codebase has to be converted first.
+
+Follow these steps to perform the ES modules conversion of a Vaadin component.
+Note that the `magi-cli` mentioned below is an internal tool used by Vaadin components team,
+in case you want to convert your own set of components, only `polymer-modulizer` is needed.
+
+```shell
+# Install global dependencies
+$ npm install -g polymer-modulizer magi-cli yarn
+
+# Commit any changes, make sure your local repository is clean
+$ git status
+
+$ magi p3-convert --out . --import-style=name
+
+# Install component dependencies from npm using yarn
+$ yarn install --flat
+
+# Run development server
+$ polymer serve --npm
+```
+
+- Open http://localhost:8081/components/@vaadin/vaadin-combo-box/test/ for the tests
+
+Note: replace `vaadin-combo-box` with the actual component in the above example.
+
+When done, return back to the HTML Imports codebase using git:
+
+```shell
+$ git reset --hard HEAD^ && git clean -df
+```
+
+### Contributing to ES module components
+
+To contribute into Polymer 3 Vaadin components, make your changes
+on the mainline Polymer 2 codebase, then open a pull request targeting
+the master branch.
+
+Note: unlike the official Polymer Elements, the converted Polymer 3 compatible
+Vaadin components are only published on npm, not pushed to GitHub repositories.
+
+### Contributor License Agreement
+
+When you send a pull request to any of our repositories, you get an automated comment response about the CLA.
+It will notify you if you haven’t signed the CLA yet, and in that case instructions how to do it.
+You need to do this once per each repository. Before we can accept any of your code contributions, you need to sign the CLA.