Write tutorial for source effect

Continue stabbing at GH-93

.gitignore

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 \#*\#
 .\#*
 *~
+
+# VS Code
+.vscode/

docs/source/tutorial/effect.rst

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+Adding an Effect
+================
+
+.. currentmodule:: palace
+
+This section will focus on how to add effects to the audio.
+
+There are two set of audio effects supported by palace: :py:class:`ReverbEffect`
+and :py:class:`ChorusEffect`.
+
+Reverb Effect
+-------------
+
+Reverb happens when a sound is reflected and then decay as the sound is absorbed
+by the objects in the medium.  :py:class:`ReverbEffect` facilitates such effect.
+
+Creating a reverb effect can be as simple as:
+
+.. code-block:: python
+
+   with ReverbEffect() as effect:
+       source.sends[0].effect = effect
+
+:py:attr:`Source.sends` is a collection of send path signals, each of which
+contains `effects` and `filter` that describes it.  Here we are only concerned
+about the former.
+
+The above code would yield a *generic* reverb effect by default.
+There are several other presets that you can use, which are listed
+by :py:data:`reverb_preset_names`.  To use these preset, you can simply provide
+the preset effect name as the first parameter for the constructor.  For example,
+to use `PIPE_LARGE` preset effect, you can initialize the effect like below:
+
+.. code-block:: python
+
+   with ReverbEffect('PIPE_LARGE') as effect:
+       source.sends[0].effect = effect
+
+These effects can be modified via their attributes.
+
+.. code-block:: python
+
+   effect.gain = 0.4
+   effect.diffusion = 0.65
+   late_reverb_pan = 0.2, 0.1, 0.3
+
+The list of these attributes and their constraints can be found
+in the documentation of :py:class:`ReverbEffect`.
+
+Chorus Effect
+-------------
+
+:py:class:`ChorusEffect` does not have preset effects like
+:py:class:`ReverbEffect`, so you would have to initialize the effect attributes
+on creation.
+
+There are five parameters to initialize the effect, respectively: waveform,
+phase, depth, feedback, and delay.
+
+.. code-block:: python
+
+   with ChorusEffect('sine', 20, 0.4, 0.5, 0.008) as effect:
+       source.sends[0].effect = effect
+
+For the constraints of these parameters, please refer to the documentation.

docs/source/tutorial/index.rst

@@ -9,7 +9,7 @@ This tutorial will guide you on:
    context
    play-audio
    source
+   effect
 .. comment these to add later
-   Adding effects
    Customize decoder
    Generate sounds