Placeholder docs for optionals

docs/optionals.md

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+# Optional Values
+
+A very common use case is values that may or may not be present. You could
+represent this case using enums like so:
+
+```tomo
+enum MaybeInt(AnInt(x:Int), NoInt)
+
+func maybe_takes_int(maybe_x:MaybeInt):
+    when maybe_x is AnInt(x):
+        say("Got an int: $x")
+    else:
+        say("Got nothing")
+```
+
+However, it's overly onerous to have to define a separate type for each
+situation where you might want to not have a value. Instead, Tomo has
+built-in support for optional types:
+
+```
+func maybe_takes_int(x:Int?):
+    if x:
+        say("Got an int: $x")
+    else:
+        say("Got nothing")
+```
+
+This establishes a common language for talking about optional values without
+having to use a more generalized form of `enum` which may have different naming
+conventions and which would generate a lot of unnecessary code.