SD library explanation (#262)

It's look more a practical guide "how to use SD lib"

libraries/SD/README.md

+
+# SD library
+
+This library provides the integration of ESP32 and SD (Secure Digital) cards without additional modules.
+
+
+## Sample wiring diagram:
+
+
+![SD card pins](http://i.imgur.com/4CoXOuR.png)
+
+For others SD formats:
+
+
+![Other SD card formats](https://upload.wikimedia.org/wikipedia/commons/thumb/a/ab/MMC-SD-miniSD-microSD-Color-Numbers-Names.gif/330px-MMC-SD-miniSD-microSD-Color-Numbers-Names.gif)
+
+
+Image source: [Wikipedia](https://upload.wikimedia.org/wikipedia/commons/thumb/a/ab/MMC-SD-miniSD-microSD-Color-Numbers-Names.gif/330px-MMC-SD-miniSD-microSD-Color-Numbers-Names.gif)
+
+```diff
+- Warning: Some ESP32 modules have different pinouts!
+```
+
+
+
+## FAQ:
+
+**Do I need any additional modules, like Arduino SD module?**
+
+No, just wire your SD card directly to ESP32.
+
+
+
+**What is the difference between SD and SD_MMC libraries?**
+
+SD runs on SPI, and SD_MMC uses the SDMMC hardware bus on the ESP32.
+
+
+
+**Can I change the CS pin?**
+
+Yes, just use: `SD.begin(CSpin)`