Add an optimization tips document. (#1024)

Motivation:

NIO doesn't offer any tips of on how to write performant code. After
discussing with @weissi we decided it would be useful to gather and
document some useful tips and tricks.

Modifications:

Started an optimization tips document.

Result:

Easier for NIO users to write performant code.

docs/optimization-tips.md

@@ -0,0 +1,33 @@
+Swift NIO Optimization Tips
+===========================
+
+The following document is a list of tips and advice for writing high performance
+code using Swift NIO.
+
+General Optimization Tips
+-------------------------
+
+Although not specific to NIO, the [Swift GitHub repository][swift-repo] has a
+list of tips and tricks for [writing high performance Swift code][swift-optimization-tips]
+and is a great place to start.
+
+Wrapping types in `NIOAny`
+--------------------------
+
+Anything passing through a NIO pipeline that is not special-cased by `NIOAny`
+(i.e. is not one of `ByteBuffer`, `FileRegion`, `IOData` or
+`AddressedEnvelope<ByteBuffer>`) needs to have careful attention paid to its
+size. If the type isn't one of the aforementioned special cases then it must be
+no greater than 24 bytes in size to avoid a heap-allocation each time it is
+added to a `NIOAny`. The size of a type can be checked using
+[`MemoryLayout.size`][docs-memory-layout].
+
+If the type being wrapped is a value type then it can be narrowed by placing it
+in a copy-on-write box. Alternatively, if the type is an `enum` with associated
+data, then it is possible for the associated data to be boxed by the compiler by
+making it an `indirect case`.
+
+
+[swift-repo]: https://github.com/apple/swift
+[swift-optimization-tips]: https://github.com/apple/swift/blob/master/docs/OptimizationTips.rst
+[docs-memory-layout]: https://developer.apple.com/documentation/swift/memorylayout