Adds missing files to the contents list in the readme. (#21)

Motivation:
To improve the visibility of recently added files.

Modifications:
Adds 3 files to the file list in README.md.
Includes additional contributor.

Result:
Easier to see the full project contents as the readme is more accurate.

CONTRIBUTORS.txt

@@ -17,3 +17,4 @@ needs to be listed here.
 - Norman Maurer <[email protected]>
 - Tom Doron <[email protected]>
 - Ludovic Dewailly <[email protected]>
+- Liam Flynn <[email protected]>

README.md

@@ -31,3 +31,6 @@ functionality.
 - [`QuiescingHelper`](Sources/NIOExtras/QuiescingHelper.swift): Helps to quiesce
   a server by notifying user code when all previously open connections have closed.
 - [`LineBasedFrameDecoder`](Sources/NIOExtras/LineBasedFrameDecoder.swift) Splits incoming `ByteBuffer`s on line endings.
+- [`FixedLengthFrameDecoder`](Sources/NIOExtras/FixedLengthFrameDecoder.swift) Splits incoming `ByteBuffer`s by a fixed number of bytes.
+- [`LengthFieldBasedFrameDecoder`](Sources/NIOExtras/LengthFieldBasedFrameDecoder.swift) Splits incoming `ByteBuffer`s by a number of bytes specified in a fixed length header contained within the buffer.
+- [`LengthFieldPrepender`](Sources/NIOExtras/LengthFieldBasedFrameDecoder.swift) Prepends the number of bytes to outgoing `ByteBuffer`s as a fixed length header. Can be used in a codec pair with the `LengthFieldBasedFrameDecoder`.