Added a module docstring

src/PyPlot.jl

@@ -1,4 +1,21 @@
 
+"""
+PyPlot allows Julia to interface with the Matplotlib library in Python, specifically the matplotlib.pyplot module, so you can create beautiful plots in Julia with your favorite Python package. 
+
+Only the currently documented matplotlib.pyplot API is exported. To use other functions in the module, you can also call matplotlib.pyplot.foo(...) as plt.foo(...). 
+For example, plt.plot(x, y) also works. (And the raw PyObject for the matplotlib modules is also accessible as PyPlot.matplotlib.)
+
+In general, all the arguments are the same as in Python. 
+
+Here's a brief demo of a simple plot in Julia:
+
+    using PyPlot
+    x = range(0; stop=2*pi, length=1000); y = sin.(3 * x + 4 * cos.(2 * x));
+    plot(x, y, color="red", linewidth=2.0, linestyle="--")
+    title("A sinusoidally modulated sinusoid")
+
+For more information on API, see the matplotlib.pyplot documentation and the PyPlot GitHub page.
+"""
 module PyPlot
 
 using PyCall