Add plugin description to index

Author: ConorMacBride

docs/index.rst

@@ -14,7 +14,11 @@
 pytest-mpl |release| documentation
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-This is a plugin to facilitate image comparison for Matplotlib figures in pytest.
+``pytest-mpl`` is a `pytest <https://docs.pytest.org>`__ plugin to facilitate image comparison for `Matplotlib <http://www.matplotlib.org>`__ figures.
+
+For each figure to test, an image is generated and then subtracted from an existing reference image.
+If the RMS of the residual is larger than a user-specified tolerance, the test will fail.
+Alternatively, the generated image can be hashed and compared to an expected value.
 
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 Installation