Add README.md.

Author: feilong

README.md

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+This simple package contains a `brain_plot` function that plots data on cortical surface.
+It assumes that:
+- The data is in fsaverage space with icoorder3 resolution (642 vertices per hemisphere).
+- The non-cortical vertices has been masked out (588 and 587 remaining vertices for the left and right hemisphere, respectively).
+
+```Python
+import numpy as np
+from brainplotlib import brain_plot
+
+# Generate some random data
+rng = np.random.default_rng(0)
+v = rng.random((1175, ))
+
+img = brain_plot(v, vmax=1, vmin=0, cmap='viridis')
+```
+
+The rendered image is a NumPy array.
+It can be rendered using `matplotlib`:
+```Python
+import matplotlib.pyplot as plt
+fig = plt.figure(figsize=(img.shape[1] / 100, img.shape[0] / 100), dpi=100)
+plt.imshow(img)
+plt.axis('off')
+plt.show()
+```
+
+Alternatively, the high-resolution image can be saved directly using `cv2`.
+```Python
+import cv2
+cv2.imwrite(
+    'random_data.png',
+    np.round(img[:, :, [2, 1, 0]] * 255).astype(np.uint8))
+```
+
+![brain image](https://github.com/feilong/brainplotlib/raw/main/images/random_data.png)