add readme with example

README.md

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+Installation
+============
+
+	pip install range-coder
+
+
+Example
+=======
+
+```python
+from range_coder import RangeEncoder, RangeDecoder, prob_to_cum_freq
+
+data = [2, 0, 1, 0, 0, 0, 1, 2, 2]
+prob = [0.5, 0.2, 0.3]
+
+# convert probabilities to cumulative integer frequency table
+cumFreq = prob_to_cum_freq(prob, resolution=128)
+
+# encode data
+encoder = RangeEncoder(filepath)
+encoder.encode(data, cumFreq)
+encoder.close()
+
+# decode data
+decoder = RangeDecoder(filepath)
+dataRec = decoder.decode(len(data), cumFreq)
+decoder.close()
+```