add copilot instructions (#11484)

Author: gewarren

.github/copilot-instructions.md

@@ -0,0 +1,17 @@
+Code comments should end with a period.
+
+When you add a code snippet to the XML remarks of an API, add the code as a separate code file (.cs file) and not as an inline (```) code block. Also add a .csproj file to compile the code if one doesn't already exist in the snippet folder.
+
+Don't use the word "may". Use "might" to indicate possibility or "can" to indicate permission.
+
+There should always be a comma before a clause that begins with "which".
+
+Use a conversational tone with contractions.
+
+Be concise.
+
+Break up long sentences.
+
+Use the present tense for instructions and descriptions. For example, "The method returns a value" instead of "The method will return a value."
+
+Use the Oxford comma in lists of three or more items.