Packages of STM32 cores are enabled for Arduino IDE.
Thanks to cooperation STMicroelectronics, stm32duino and Arduino is possible download packages for Nucleo-F091RC, Nucleo-F103RB, Nucleo-L476RG.
Further support of another microcontrollers will surely follow.

How to do it?
Download Arduino IDE.

Add json file https://github.com/stm32duino/BoardManagerFiles/raw/master/STM32/package_stm_index.json to Preferences -> Additional Boards Managers URLs

go to Boards Manager, find STM32 and install package of your board.

In Tools -> Board choose STM32 board. That's all.

For uploading of your code you have three options. Use mass storage (copy bin file to mass storage), use STLink (STLink to FLASH) or DFU (USB to FLASH).

News here https://community.st.com/community/stm32-community/blog/2017/02/02/stm32-cores-enabled-in-arduino-ide

Getting started here https://github.com/stm32duino/wiki/wiki/Getting-Started