add a readme

Author: lukasmwerner

README.md

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+# Py-Image-Diff
+
+A simple python image difference library. It returns the % difference of the
+image. Additionally, it also saves the visual differences to a file called
+`diff.png`.
+
+Here is a sample image of the visual difference:
+
+![](./diff.png)
+
+### Usage
+
+To use the library you need to build the `.so`. Here is the go building command:
+`go build -o lib.so -buildmode=c-shared library.go`
+
+Usage of the library is really simple. The only exported function is `compare`.
+
+```python
+import imagediff
+
+with open("a.png", 'rb') as a:
+    with open("b.png", 'rb') as b:
+        difference = library.compare(a.read(), b.read())
+        print(difference)
+```