Add initial bbt markers documentation

include/bbt-markers.html

@@ -0,0 +1,28 @@
+<p>
+  <dfn>BBT markers</dfn> allow resetting the timeline to a particular bar and
+  beat. Most of the time a BBT marker will reset it to 1|1|0 (the very first
+  beat of the very first bar) so that for each song in a multi-song session
+  there would be a musical time origin. However it's entirely possible to reset
+  to an arbitrary value such as 7|3|0.
+</p>
+
+<p>
+  To create a new BBT marker, <kbd class="mod1n"></kbd>-click on the
+  Bars:Beats ruler. This will open a dialog titled 'New Music Time' where you
+  can name the marker and define what bar (left field) and beat (right field)
+  this marker should reset the time to.
+</p>
+
+<figure>
+  <img style="width:50%;" width="50%" src="/images/new-music-time.png" alt="New Music Time dialog">
+  <figcaption>New Music Time dialog</figcaption>
+</figure>
+
+<p>
+  The timeline grid and the ruler will be updated accordingly:
+</p>
+
+<figure>
+  <img style="width:50%;" width="50%" src="/images/bbt-marker-resets-musical-time.png" alt="The BBT marker resets the musical time">
+  <figcaption>The BBT marker resets the musical time</figcaption>
+</figure>

master-doc.txt

@@ -1652,6 +1652,12 @@ link: techniques-for-working-with-tempo-and-meter
 part: subchapter
 ---
 
+---
+title: BBT markers
+include: bbt-markers.html
+link: bbt-markers
+part: subchapter
+---
 
 ---
 title: Mixing