Firmware docs 0.5.0

[m-mcgowan]

New docs for 0.5.0

• new SOS code for Stack overflow @m-mcgowan
• cellular band API Cellular Band Select API device-os#891 @technobly
• cellular data usage API adds Cellular.dataUsage() API device-os#866 @technobly
• cellular localIP Cellular.localIP() API device-os#850 @technobly
• Serial.availableForWrite() / Serial.blockOnOverrun() Serial.availableForWrite() and Serial.blockOnOverrun() implementation, USB Serial fixes for STM32F2 device-os#812 @avtolstoy availableForWrite() and blockOnOverrun() are available on USB Serial since 0.5.0 #339
• System.sleep(SLEEP_NETWORK_STANDBY) @m-mcgowan
• System.sleep(pin, edge, duration) on wakeup, connects to the cloud in AUTOMATIC/SEMI-AUTOMATIC mode. (Not when threading is enabled. To wait for the cloud to be reconnected, e.g. to publish immediately after waking, use waitFor(Particle.connected) @m-mcgowan
• SPI Slave - SPI Slave implementation for Photon and Electron (STM32F2) device-os#882 @avtolstoy SPI slave documentation #338
• EEPROM emulation revamp - Update EEPROM documentation for atomic operations #323 Implement atomic writes for EEPROM emulation device-os#871 @monkbroc
• long variable/function names compiler error - Give a compiler error for long names in Particle.variable/function device-os#883 @monkbroc
• Publish NO_ACK - Feature/publish noack device-os#862 @m-mcgowan
• configurable keep alive - configurable keepalive device-os#913 @m-mcgowan
• serial parity - Adds docs for serial.begin() usart config settings #317
• Application Watchdog - Feature/app watchdog device-os#860 @m-mcgowan
• measure and add Particle.keepAlive() data usage - configurable keepalive device-os#913 @technobly
• application softAP pages - application softAP http pages device-os#906 @m-mcgowan
• Timer.isActive() - Timer::isActive() device-os#950 @sergeuz
  When the location of the docs for a feature is known, please link from the corresponding entry in CHANGELOG.md in the firmware repo.