Document "b" (breakpoint) event type

doc/asciicast-v2.md

@@ -13,7 +13,8 @@ Example file:
 {"version": 2, "width": 80, "height": 24, "timestamp": 1504467315, "title": "Demo", "env": {"TERM": "xterm-256color", "SHELL": "/bin/zsh"}}
 [0.248848, "o", "\u001b[1;31mHello \u001b[32mWorld!\u001b[0m\n"]
 [1.001376, "o", "That was ok\rThis is better."]
-[2.143733, "o", " "]
+[1.500000, "b", ""]
+[2.143733, "o", "Now... "]
 [6.541828, "o", "Bye!"]
 ```
 
@@ -114,7 +115,7 @@ Where:
 
 * `time` (float) - indicates when this event happened, represented as the number
   of seconds since the beginning of the recording session,
-* `event-type` (string) - one of: `"o"`, `"i"`,
+* `event-type` (string) - one of: `"o"`, `"i"`, `"b"`
 * `event-data` (any) - event specific data, described separately for each event
   type.
 
@@ -164,6 +165,18 @@ non-printable Unicode codepoints encoded as `\uXXXX`.
 > implementations of asciicast-compatible terminal recorder should not capture
 > it either unless explicitly permitted by the user.
 
+#### "b" - breakpoint
+
+Event of type `"b"` represents a playback breakpoint.
+
+When breakpoint is encountered in the event stream and "pause on breakpoint"
+functionality of the player is enabled, the playback should pause, and wait for
+the user to resume.
+
+`event-data` can be used to annotate (comment) a breakpoint. Annotations may be
+used to show a list of "chapters" (players that load whole recording into
+memory only).
+
 ## Notes on compatibility
 
 Version 2 of asciicast file format solves several problems which couldn't be