add README

Author: adusachev

README.md

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+# QOI image compression
+
+Python implementation of QOI encoder and decoder
+
+QOI format: https://qoiformat.org/
+
+
+## Usage
+
+**1) Encode single .png image and save .qoi image**
+
+At the end of `qoi_encoder.py` enter path to .png image and run: 
+```
+python3 qoi_encoder.py
+```
+
+
+**2) Decode single .qoi image**
+
+At the end of `qoi_decoder.py` enter path to .qoi image and run: 
+```
+python3 qoi_decoder.py
+```
+
+
+**3) Encode and decode all .png images in some directory, compare decoded .qoi image with original .png image**
+
+At the end of `main.py` choose `dir_with_png` and uncomment line with `run_multiple_experiments(dir_with_png)`, then run:
+```
+python3 main.py
+```
+