Added PR message to BME280 usermod as readme

usermods/BME280_v2/README.md

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+Hello! I have written a v2 usermod for the BME280/BMP280 sensor based on the [existing v1 usermod](https://github.com/Aircoookie/WLED/blob/master/usermods/Wemos_D1_mini%2BWemos32_mini_shield/usermod_bme280.cpp). It is not just a refactor, there are many changes which I made to fit my use case, and I hope they will fit the use cases of others as well! Most notably, this usermod is *just* for the BME280 and does not control a display like in the v1 usermod designed for the WeMos shield. 
+
+- Requires libraries `BME280@~3.0.0` (by [finitespace](https://github.com/finitespace/BME280)) and `Wire`. Please add these under `lib_deps` in your `platform.ini` (or `platform_override.ini`).
+- Data is published over MQTT so make sure you've enabled the MQTT sync interface.
+- This usermod also writes to serial (GPIO1 on ESP8266). Please make sure nothing else listening on the serial TX pin of your board will get confused by log messages!
+
+To enable, compile with `USERMOD_BME280` defined (i.e. `platformio_override.ini`)
+```ini
+build_flags =
+  ${common.build_flags_esp8266}
+  -D USERMOD_BME280
+```
+or define `USERMOD_BME280` in `my_config.h`
+```c++
+#define USERMOD_BME280
+```
+
+Changes include:
+- Adjustable measure intervals
+  - Temperature and pressure have separate intervals due to pressure not frequently changing at any constant altitude
+- Adjustment of number of decimal places in published sensor values
+  - Separate adjustment for temperature, humidity and pressure values
+  - Values are rounded to the specified number of decimal places
+- Pressure measured in units of hPa instead of Pa
+- Calculation of heat index (apparent temperature) and dew point
+  - These, along with humidity measurements, are disabled if the sensor is a BMP280
+- 16x oversampling of sensor during measurement
+- Values are only published if they are different from the previous value
+- Values are published on startup (continually until the MQTT broker acknowledges a successful publication)
+
+Adjustments are made through preprocessor definitions at the start of the class definition.
+
+MQTT topics are as follows:
+Measurement type | MQTT topic
+--- | ---
+Temperature | `<deviceTopic>/temperature`
+Humidity | `<deviceTopic>/humidity`
+Pressure | `<deviceTopic>/pressure`
+Heat index | `<deviceTopic>/heat_index`
+Dew point | `<deviceTopic>/dew_point`