[librsvg: 9/10] Add mask-type to FEATURES.md

[Marge Bot <marge-bot src gnome org>]

commit 9cae2dcdbb2f706918297cdef8062ede70080335
Author: Federico Mena Quintero <federico gnome org>
Date:   Fri Oct 15 18:53:13 2021 -0500

    Add mask-type to FEATURES.md
    
    Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/609>

 FEATURES.md                     |  1 +
 devel-docs/adding-a-property.md | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)
---
diff --git a/FEATURES.md b/FEATURES.md
index d8286c96..56385938 100644
--- a/FEATURES.md
+++ b/FEATURES.md
@@ -318,6 +318,7 @@ which properties are not available as presentation attributes.
 | marker-mid                  |                                                        |
 | marker-start                |                                                        |
 | mask                        |                                                        |
+| mask-type                   |                                                        |
 | mix-blend-mode              | Not available as a presentation attribute.             |
 | opacity                     |                                                        |
 | overflow                    |                                                        |
diff --git a/devel-docs/adding-a-property.md b/devel-docs/adding-a-property.md
index 531c766f..90f953e2 100644
--- a/devel-docs/adding-a-property.md
+++ b/devel-docs/adding-a-property.md
@@ -564,3 +564,24 @@ the reference, and compares their rendered results.  It also takes a test name
 
 ## Final steps: documentation
 
+To help people who are wondering what SVG features are supported in librsvg, there is a
+`FEATURES.md` file.  It has a section called "CSS properties" with a big list of property
+names and notes about them.
+
+We'll patch it like this:
+
+```
+ | marker-mid                  |                                                        |
+ | marker-start                |                                                        |
+ | mask                        |                                                        |
++| mask-type                   |                                                        |
+ | mix-blend-mode              | Not available as a presentation attribute.             |
+ | opacity                     |                                                        |
+ | overflow                    |                                                        |
+```
+
+There is nothing remarkable about `mask-type`, it is a plain old property that also has a
+presentation attribute (remember the `PresentationAttr::Yes` from above?), so we don't
+need to list any extra information.
+
+And with that, we are done implementing `mask-type`.  Have fun!