Update README.

Author: brocaar

README.md

@@ -65,6 +65,58 @@ configuration.
 This directory contains the profiles. These profiles can be used by one
 or multiple devices. The profile also defines the region.
 
+## Adding a new device-profile
+
+In summary, these are the steps to create a new device-profile. 
+
+1. **Add a new vendor directory in `vendors/`.** This directory should be
+   lower-cased, and spaces must be replaced by `-`. For example for
+   _My Vendor_ you would create a directory `vendors/my-vendor/`.
+
+2. **Create `vendor.toml` file.** Inside the directory created in the previous
+   step, create a `vendor.toml` file (e.g. `vendors/my-vendor/vendor.toml`).
+   Please use `vendors/example-vendor/vendor.toml` as an example. Under `devices`
+   you want to refer to the devices (that you will add in the next steps).
+   If you have a temperature and a humidity sensor device (thus two devices),
+   your `devices` configuration could look like:
+   `devices = ["temperature.toml", "humidity.toml"]`
+
+3. **Create device file(s).** In this step you need to create a file for each
+   device that you configured in the previous step under `devices`. To
+   continue with the previous example, you would create two files:
+   * `vendors/my-vendor/devices/temperature.toml`
+   * `vendors/my-vendor/devices/humidity.toml`
+   You can use `vendors/example-vendor/devices/example.toml` as an example.
+   For each firmware version (if there are multiple), make sure to also
+   configure the `profiles` and `codec` options (if you provide a payload)
+   codec. For example your `temperature.toml` device configuration could
+   contain `profiles= ["temerature-eu868.toml", "temperature-us915.toml"]` and
+   `codec = "temperature.js"` if there a a codec for this device.
+
+4. **Create profile file(s).** In this step you need to create a file for
+   each profile that you configured in the previous step for your device(s).
+   To continue with the previous example, you would create:
+   * `vendors/my-vendor/profiles/temperature-eu868.toml`
+   * `vendors/my-vendor/profiles/temperature-us915.toml`
+   You can use `vendors/example-vendor/profiles/example-EU868.toml` as an
+   example.
+
+5. **Create codec file(s).** If you have configured any `codec` options, you
+   must create these codec files. To continue with the previous example, you
+   would create a file named `vendors/my-vendor/codecs/temperature.js`. You
+   can use `vendors/example-vendor/codecs/example.js` as an example.
+
+6. **Add codec tests (optional).** To validate that the codec returns the
+   expected output given a certain input you can add tests to your codecs.
+   To continue with the previous example, you would create two files:
+   * `vendors/my-vendor/codecs/test_decode_temperature.json`
+   * `vendors/my-vendor/codecs/test_encode_temperature.json`
+   Please see `test_decode_example.json` and `test_encode_example.json` in 
+   the `vendors/example-vendor/codecs` directory.
+
+7. **Run tests.** This validates that the above configuration can be parsed
+   correctly and that the codec return the expected output.
+
 ## Running the test
 
 To run the test-runner, execute: