Add a bunch of doc comments

We get a lot of build output because of missing XML comments that we
ignore. I'd like to stop ignoring them. To do that, we need to doc the
💩 out of everything.

Octokit/Models/Request/NewTree.cs

@@ -6,6 +6,13 @@ using System.Globalization;
 
 namespace Octokit
 {
+    /// <summary>
+    /// Used to create a new Tree.
+    /// </summary>
+    /// <remarks>
+    /// The tree creation API will take nested entries as well. If both a tree and a nested path modifying that tree 
+    /// are specified, it will overwrite the contents of that tree with the new path contents and write a new tree out.
+    /// </remarks>
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class NewTree
     {