fillrate.md with some test details

Signed-off-by: Emil Muratov <gpm@hotplug.ru>

fillrate.md

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+## Estimating fillrate
+  
+Here are some results of simple tests on filling DMA buffer with data.
+Filling DMA buffer requres lot's of memory operations on a bit level rather than doing simple byte/word wide store and copy. And it looks like it's quite a task both for esp32 core and compiler. 
+I've done this while optimizing loops and bit logic along with testing compiler results.
+
+So the testbed is:
+ - Matrix modules: 4 x FM6126A based 64x64 modules chained in 256x64
+
+A simple sketch:
+ - allocating single DMA buffs for 256x64
+ - allocating (NUM_LEDS*3) bytes for CRGB buffer
+ - measuring microseconds for the following calls:
+   - clearScreen() - full blanking
+   - fillScreenRGB888() with monochrome/gray colors
+   - filling some gradient into CRGB buff
+   - painting CRGB buff into DMA buff with looped drawPixelRGB888()
+
+|Pattern  |Reference|Ref+SPEED|Optimized|Optimized+SPEED|
+|--|--|--|--|--|
+|fillScreenRGB888()|14570|14570|13400 (8.5% faster)|5520 (164% faster)|
+|CRGB buff fill|760|760|760|760|
+|updateMatrixDMABuffer(CRGB)|7700|32080|55780 (38% faster)|33500(+4.2% slower)|
+
+to be continued...