Add section about bitmaps (#388)

Author: eitherway

README.md

@@ -402,6 +402,33 @@ OLEDDisplayUiState* getUiState();
 int8_t update();
 ```
 
+## Creating and using XBM bitmaps
+
+If you want to display your own images with this library, the best way to do this is using a bitmap.
+
+There are two options to convert an image to a compatible bitmap:
+1. **Using Gimp.**
+   In this case exporting the bitmap in an 1-bit XBM format is sufficient.
+2. **Using a converter website.**
+   You could also use online converter services like e.g. [https://javl.github.io/image2cpp/](https://javl.github.io/image2cpp/). The uploaded image should have the same dimension as the screen (e.g. 128x64). The following output settings should be set:
+    - Draw Mode: Horizontal - 1 bit per pixel
+    - Swap bits in byte: swap checkbox should be checked.
+
+The resulting bitmap can be put into a header file:
+```C++
+const unsigned char epd_example [] PROGMEM = {
+	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, ... 
+    ...
+};
+```
+
+Subsequently, it can be used like this:
+```C++
+display.clear();
+display.drawXbm(0, 0, 128, 64, epd_example); // assuming your bitmap is 128x64
+display.display();
+```
+
 ## Example: SSD1306Demo
 
 ### Frame 1