add custom field.

F1.md

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+NIP-F1
+======
+
+Profile Hypercustomization
+--------------------------
+
+`draft` `optional`
+
+This NIP describes a new event `kind:19999` that can be used for multiple optional and weird forms of profile customization, such as extra colors, extra pictures, preferences and background music.
+
+It's completely optional and clients may choose to fulfill only some of these tags, or none. Clients may also allow users to turn off these customizations or not.
+
+## Tags
+
+| Name               | Value type                         | Description                                                                             |
+| ---                | ----                               | -----------                                                                             |
+| `background-color` | hex value                          | To be used by clients when displaying the user profile page                             |
+| `foreground-color` | hex value                          | Idem                                                                                    |
+| `background-music` | URL                                | Music that optionally plays when the profile is opened                                  |
+| `priority_kinds`   | stringified kind number (variadic) | Kinds to be displayed by default in the user profile page (rather than `kind:1` always) |
+| `custom`           | string name, then value            | An arbitrary field to be displayed along with the profile                               |
+
+## Example
+
+```yaml
+{
+  "kind": 19999,
+  "tags": [
+    ["background-color", "#1a1a2e"],
+    ["foreground-color", "#eee444"],
+    ["background-music", "https://example.com/music/profile-theme.mp3"],
+    ["priority_kinds", "20", "30023", "21", "10009"],
+    ["custom", "favorite fruit", "banana"],
+    ["custom", "pets?", "no"]
+  ],
+  // ...other fields
+}
+```