Created smnp.music.command (markdown)

smnp.music.command.md

@@ -0,0 +1,45 @@
+This module contains a common commands and command-like functions that can be shared between available music services (like [[smnp.audio.midi]] or [[smnp.audio.synth]]) and are supported by all of them. 
+
+# Functions
+## velocity
+```php
+velocity(velocity: float)
+```
+The factory function for `velocity` command, which changes the velocity of notes.
+
+#### Arguments
+* `velocity ` - the velocity-value of notes between `0.0` and `1.0`
+
+#### Output
+* a command map
+
+## cresc
+```php
+cresc(begin: float, end: float, ...notes: <note, int>)
+cresc(begin: float, end: float, notes: list<note, int>)
+```
+Applies a `crescendo` effect on provided notes.
+
+#### Arguments
+* `begin` - the beginning velocity value for the first note
+* `end` - the ending velocity value for the last note
+* `notes` - the notes on which the `crescendo` is intended to be applied.
+
+#### Crescendo or diminuendo
+The function allows you to work with two modes:
+* _crescendo_ if `begin` < `end`
+* _diminuendo_ if `begin` > `end`
+
+#### Output
+* a list of list contained of provided notes delimited by `velocity` command of value linearly-interpolated between `begin` for the first note of `notes` and `end` for the last note
+
+#### Example
+```php
+pp = 0.2;
+mf = 0.7;
+x = $ cresc(pp, mf, @c, @d, @e, @f, @g, @a, @h, @c5) ||.flatten();
+# Note, that the `cresc()` function produces a list which is typically being nested into staff. 
+# Because of that there could be a need to make the staff flat using i.e. `flatten()` method on it
+# from `smnp.collection` module.
+```
+