Update README.md

added some description

Author: JojoS62

README.md

@@ -7,3 +7,22 @@ This is not officially supported driver.
 ## License & Contributions 
 
 The software is provided under [Apache-2.0 license](LICENSE). Contributions to this project are accepted under the same license. Please see [contributing.md](CONTRIBUTING.md) for more info.
+
+This is an implementation for a blockdevice to use SDHC cards via SDIO interface.
+
+## Supported Hardware
+- STM32F4 
+
+## Tested Hardware
+- STM Discovery board 32F469IDISCOVERY, Targetname: DISCO_F469NI
+  https://os.mbed.com/platforms/ST-Discovery-F469NI/
+  
+  
+## Driver Description
+This SDIOBlockdevice inherits from Blockdevice. The implementation uses an intermediate layer for the target specific code. This version uses the STM HAL 
+for the SD card communication, an approach that is not fully mBed compliant. So this driver is not part of the mbed-os, a cleaner solution maybe supplied in the future. Anyway, this driver passes the same tests as for a SDBlockdevice with SPI interface.
+
+## Usage
+The driver is used like other blockdevices, a good starting point is https://github.com/ARMmbed/mbed-os-example-filesystem
+
+