Add ML.NET notes from session 5

2018/Microsoft.ML.Core/Session5.md

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+# Microsoft.ML
+
+Status: **Needs more work**
+
+## Notes
+
+* `MLContext`
+    - Solves two problems: the chaining for `IHostEnvironment` and the
+      discoverability for finding implementations of common abstraction.
+    - They are solved by offering extension methods
+    - We should align the naming of `XxxCatalog` with `XxxContext`
+    - It's a bit odd that the constructor uses nullability to indicate default,
+      but sentinel values seem more problem. Overloads don't work very well
+      because the types are the same and it will force people to specify value
+      of the other. Easiest solution might be to make both nullable. Still not
+      idiomatic API design, but at least consistent.
+* [Typical example](https://dotnet.microsoft.com/learn/machinelearning-ai/ml-dotnet-get-started-tutorial)
+    - Most of the methods aren't verbs; instead of `TextReader()` it should be
+      `CreateTextReader()`
+    - We should have a pattern for the arguments:
+        - Simple set of arguments should just be parameters
+        - More complicated arguments should be an option class that is mutable
+        - Some types probably want to offer discrete parameters for convenience
+          while also being able to accept more complex options. The overloads
+          should never mix them as this would create conflicts which value
+          "wins".
+        - Example:
+            ```C#
+            var options = new TextFileSource.Options();
+            options.Separator = ",",
+            options.HasHeader = true,
+            options.Column.Add("SepalLength", DataKind.R4, 0);
+            options.Column.Add("SepalWidth", DataKind.R4, 1);
+            options.Column.Add("PetalLength", DataKind.R4, 2);
+            options.Column.Add("PetalWidth", DataKind.R4, 3);
+            options.Column.Add("Label", DataKind.Text, 4);
+
+            var reader = mlContext.Data.CreateTextFileSource(options);
+            ```
+    - Tom check what you were plan on checking :-)