Update ml.md
initial work on an ml chapter for user guide
+# Machine Learning
+
+## What is ML?
+Machine learning is one of a group of synonomous or near synonomous names (like, data mining, or predictive modeling)
+applied to an approach to building mathematical models from data.
+
+There are two kinds, broadly speaking. One kind is called "supervised" learning models, and the other "unsupervised".
+
+Always we have a set of data, organized in rows and columns, where each row represents a single "case" in the domain of interest.
+Each column contains a value representing an attribute of the case we might wish to include in our model. One of these may
+be a thing we want to predict about each case, sometimes called the response variable. The others are potential
+"predictor" variables.
+
+Supervised methods use a training set of data where the predicted value for each case is known.
