Add simple PINN example

Author: gjbex

source-code/README.md

@@ -32,3 +32,5 @@ to create it.  There is some material not covered in the presentation as well.
   the curse of dimensionality.
 * [`lime`](lime/): illustration of using LIME to explain machine learning
   models.
+* [`pinns`](pinns/): illustrations of using PINNs to solve mathematical and physics
+  problems.

source-code/pinns/README.md

@@ -0,0 +1,11 @@
+# PINNs
+
+Physics-Informed Neural Networks (PINNs) are (deep) neural networks that are
+trained using physics properties, e.g., ordinary or partial differential
+equations in the loss function.
+
+
+## What is it?
+
+1. `logistic_de.ipynb`: Jupyter notebook solving the logistic differential
+   equation using a PINN.