add HTU21D sensor platform doc (#2820)

* add HTU21D sensor platform doc

* don't repeat errors!

* review fixes

source/_components/sensor.htu21d.markdown

@@ -0,0 +1,106 @@
+---
+layout: page
+title: "HTU21D Temperature and humidity sensor"
+description: "Instructions how to integrate a HTU21D Temperature and humidity sensor in a Raspberry PI into Home Assistant."
+date: 2017-06-10 00:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: raspberry-pi.png
+ha_category: Sensor
+ha_release: 0.47
+ha_iot_class: "Local Push"
+---
+
+
+The `htu21d` sensor platform allows you to read the temperature and humidity from a [HTU21D sensor](http://www.datasheetspdf.com/PDF/HTU21D/779951/1) connected via [I2c](https://en.wikipedia.org/wiki/I²C) bus (SDA, SCL pins) to your [Raspberry Pi](https://www.raspberrypi.org/).
+
+To use your HTU21D sensor in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: htu21d
+```
+
+Configuration variables:
+
+- **name** (*Optional*): The name of the sensor
+- **i2c_bus** (*Optional*): I2c bus where the sensor is. Defaults to 1, for Raspberry Pi 2 and 3.
+
+#### Customizing the sensor data
+
+**Give the values friendly names & icons**
+
+Add the following to your `customize`
+
+```yaml
+# Example configuration.yaml entry
+customize:
+  sensor.htu21d_sensor_temperature:
+    icon: mdi:thermometer
+    friendly_name: "Temperature"
+  sensor.htu21d_sensor_humidity:
+    icon: mdi:weather-rainy
+    friendly_name: "Humidity"
+```
+
+**Create a group**
+
+Add the following to your `groups`
+
+```yaml
+# Example configuration.yaml entry
+group:
+  ambient_sensor:
+    name: HTU21D Environment sensor
+    entities:
+      - sensor.htu21d_sensor_temperature
+      - sensor.htu21d_sensor_humidity
+```
+
+### Directions for installing smbus support on Raspberry Pi:
+
+Enable I2c interface with the Raspberry Pi config utility:
+```bash
+# pi user environment: Enable i2c interface
+sudo raspi-config
+```
+Select `Interfacing options->I2C` choose `<Yes>` and hit `Enter`, then go to `Finish` and you'll be prompted to reboot.
+
+Install dependencies for use the `smbus-cffi` module and enable your _homeassistant_ user to join the _i2c_ group:
+```bash
+# pi user environment: Install i2c dependencies and utilities
+sudo apt-get install build-essential libi2c-dev i2c-tools python-dev libffi-dev
+
+# pi user environment: Add homeassistant user to the i2c group
+sudo addgroup homeassistant i2c
+
+# pi user environment: Reboot Raspberry Pi to apply changes
+sudo reboot
+```
+
+###### Check the i2c address of the sensor
+
+After installing `i2c-tools`, a new utility is available to scan the addresses of the connected sensors:
+
+```bash
+/usr/sbin/i2cdetect -y 1
+```
+
+It will output a table like this:
+
+```text
+     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
+00:          -- -- -- -- -- -- -- -- -- -- -- -- --
+10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+20: -- -- -- 23 -- -- -- -- -- -- -- -- -- -- -- --
+30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+40: 40 -- -- -- -- -- UU -- -- -- -- -- -- -- -- --
+50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+70: -- -- -- -- -- -- -- 77
+```
+
+So you can see the sensor is present at the **0x40** address (there are more i2c sensors in that Raspberry Pi).