Add "how to use" section

README.md

@@ -4,3 +4,20 @@ Cube2Cyl
 A panoramic lib, cubic to cylindrical projection conversion
 
 Read more details of implementation here: http://www.wenyanan.com/cube2cyl/
+
+How to use
+----------
+
+The Cube2Cyl library is a single header file library, written in C++. There is a demo application included as usage example.
+
+The library takes the following parameters:
+
+    Width of the panorama (must be lager than 1)
+    Height of the panorama (must be lager than 1)
+    Width of the square input (the input must have a ratio of 1:1)
+    View portion horizontally, [0.01, 2π]
+    View portion vertically, [0.01, π]
+
+This library takes any valid parameter and deals the ratio changes automatically. You can use super sampling to generate accurate panorama.
+
+For the maximum performance, using a precomputed map is recommended rather than doing the calculation for each frame.