Add a diagram of ReadBuf layout

Author: sfackler

text/0000-read-buf.md

@@ -252,6 +252,13 @@ impl Read for TcpStream {
 /// buffer that has been logically filled with data, a region that has been initialized at some point but not yet
 /// logically filled, and a region at the end that is fully uninitialized. The filled region is guaranteed to be a
 /// subset of the initialized region.
+///
+/// In summary, the contents of the buffer can be visualized as:
+/// ```not_rust
+/// [             capacity              ]
+/// [ filled |         unfilled         ]
+/// [    initialized    | uninitialized ]
+/// ```
 pub struct ReadBuf<'a> {
     buf: &'a mut [MaybeUninit<u8>],
     filled: usize,