docs: create CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing to Eclipse Thingweb
+
+Thanks for your interest in this project. General information
+regarding source code management, builds, coding standards, and
+more can be found here:
+
+-   https://projects.eclipse.org/projects/iot.thingweb/developer
+
+## Legal Requirements
+
+Thingweb is an [Eclipse IoT](https://iot.eclipse.org) project and as such is governed by the Eclipse Development process.
+This process helps us in creating great open source software within a safe legal framework.
+
+Thus, before your contribution can be accepted by the project team, contributors must electronically sign the [Eclipse Contributor Agreement (ECA)](http://www.eclipse.org/legal/ECA.php) and follow these preliminary steps:
+
+-   Obtain an [Eclipse Foundation account](https://accounts.eclipse.org/)
+    -   Anyone who currently uses Eclipse Bugzilla or Gerrit systems already has one of those
+    -   Newcomers can [create a new account](https://accounts.eclipse.org/user/register?destination=user)
+-   Add your GiHub username to your Eclipse Foundation account
+    -   ([Log into Eclipse](https://accounts.eclipse.org/))
+    -   Go to the _Edit Profile_ tab
+    -   Fill in the _GitHub ID_ under _Social Media Links_ and save
+-   Sign the [Eclipse Contributor Agreement](http://www.eclipse.org/legal/ECA.php)
+    -   ([Log into Eclipse](https://accounts.eclipse.org/))
+    -   If the _Status_ entry _Eclipse Contributor Agreement_ has a green checkmark, the ECA is already signed
+    -   If not, go to the _Eclipse Contributor Agreement_ tab or follow the corresponding link under _Status_
+    -   Fill out the form and sign it electronically
+-   Sign-off every commit using the same email address used for your Eclipse account
+    -   Set the Git user email address with `git config user.email "<your Eclipse account email>"`
+    -   Add the `-s` flag when you make the commit(s), e.g. `git commit -s -m "feat: add support for magic"`
+-   Open a [Pull Request](https://github.com/eclipse-thingweb/node-wot/pulls)
+
+For more information, please see the Eclipse Committer Handbook:
+https://www.eclipse.org/projects/handbook/#resources-commit
+
+## Contact
+
+Contact the project developers via the project's "dev" list.
+
+-   https://dev.eclipse.org/mailman/listinfo/thingweb-dev