BMP280 driver: Add README.md

extras/bmp280/README.md

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+# Driver for BMP280 absolute barometric pressure sensor
+
+The driver works only with BMP280 sensor. For BMP080/BMP180 there's a separate
+driver. Even though BMP280 is a successor of BMP180 they are not compatible.
+They have different registers and different operation modes.
+BMP280 supports two ways of communication: spi and i2c.
+This driver provides only i2c communication.
+The driver is written for [esp-open-rtos](https://github.com/SuperHouse/esp-open-rtos)
+framework and requires [i2c driver](https://github.com/SuperHouse/esp-open-rtos/tree/master/extras/i2c)
+from it.
+
+## Features
+
+ * I2C communication.
+ * Forced mode (Similar to BMP180 operation).
+ * Normal mode. Continuous measurement.
+ * Soft reset.
+
+## Usage
+
+Connect BMP280 module to you ESP8266 module and specify SCL and SDA pins:
+
+```
+const uint8_t scl_pin = 5;
+const uint8_t sda_pin = 4;
+```
+
+Pull up SDO pin of BMP280 in order to have address 0x77.
+Or pull down SDO pin and change `#define BMP280_ADDRESS 0x77` to
+`#define BMP280_ADDRESS 0x76`. Otherwise your sensor will not work.
+By default address 0x77 is used, so SDO pin should be high.
+
+BMP280 supports two operation modes.
+
+### Forced mode
+
+In forced mode, a single measurement is performed according to selected
+configuration. When the measurement is finished, the sensor returns to
+sleep mode and the measurement results can be read.
+
+### Normal mode
+
+Normal mode continuously cycles between measurement period and standby period,
+whose time is defined by standby_time.
+
+## Example
+
+### Forced mode
+
+```
+const uint8_t scl_pin = 5;
+const uint8_t sda_pin = 4;
+bmp280_params_t  params;
+float pressure, temperature;
+
+bmp280_init_default_params(&params);
+params.mode = BMP280_MODE_FORCED;
+bmp280_init(&params, scl_pin, sda_pin);
+
+bmp280_force_measurement();
+
+while (bmp280_is_measuring()) {}; // wait for measurement to complete
+
+bmp280_read(&temperature, &pressure);
+printf("Pressure: %.2f Pa, Temperature: %.2f C\n", pressure, temperature);
+```
+
+### Normal mode
+
+```
+bmp280_params_t  params;
+float pressure, temperature;
+
+bmp280_init_default_params(&params);
+bmp280_init(&params, scl_pin, sda_pin);
+
+bmp280_read(&temperature, &pressure);
+printf("Pressure: %.2f Pa, Temperature: %.2f C\n", pressure, temperature);
+
+```
+
+## License
+
+The driver is released under MIT license.
+Copyright (c) 2016 sheinz (https://github.com/sheinz)