Add Emission Shapes info to Particles2D tutorial

tutorials/2d/particle_systems_2d.rst

@@ -166,8 +166,6 @@ means particles are drawn according to their emission order (default).
 ParticlesMaterial settings
 --------------------------
 
-Particles2D > Process Material > Material > drop-down menu > Edit:
-
 .. Commented out as not implemented in 3.x for now.
 ..
    Direction
@@ -285,3 +283,56 @@ Hue variation
 The ``Variation`` value sets the initial hue variation applied to each
 particle. The ``Variation Random`` value controls the hue variation
 randomness ratio.
+
+Emission Shapes
+---------------
+
+ParticlesMaterials allow you to set an Emission Mask, which dictates 
+the area and direction in which particles are emitted. 
+These can be generated from textures in your project.
+
+Ensure that a ParticlesMaterial is set, and the Particles2D node is selected. 
+A "Particles" menu should appear in the Toolbar:
+
+.. image:: img/emission_shapes1.png
+
+Open it and select "Load Emission Mask":
+
+.. image:: img/emission_shapes2.png
+
+Then select which texture you want to use as your mask:
+
+.. image:: img/emission_shapes3.png
+
+A dialog box with several settings will appear.
+
+Emission Mask
+~~~~~~~~~~~~~
+
+Three types of emission masks can be generated from a texture:
+
+-  Solid Pixels: Particles will spawn from any area of the texture,
+   excluding transparent areas.
+.. image:: img/emission_mask_solid.gif
+-  Border Pixels: Particles will spawn from the outer edges of the texture.
+.. image:: img/emission_mask_border.gif
+-  Directed Border Pixels: Similar to Border Pixels, but adds
+   extra information to the mask to give particles the ability to emit
+   away from the borders. Note that an ``Initial Velocity`` will need to 
+   be set in order to utilize this.
+.. image:: img/emission_mask_directed_border.gif
+
+Emission Colors
+~~~~~~~~~~~~~~~
+
+``Capture from Pixel`` will cause the particles to inherit the color of the mask at their spawn points.
+
+Once you click "OK", the mask will be generated and set to the ParticlesMaterial, under the ``Emission Shape`` section:
+
+.. image:: img/emission_shapes4.png
+
+All of the values within this section have been automatically generated by the 
+"Load Emission Mask" menu, so they should generally be left alone.
+
+.. note:: An image should not be added to ``Point Texture`` or ``Color Texture`` directly.
+          The "Load Emission Mask" menu should always be used instead.