Add documentation for MediaPlaceholder props.

packages/block-editor/src/components/media-placeholder/README.md

@@ -27,6 +27,92 @@ const { MediaPlaceholder } = wp.editor;
 	}
 ```
 
+## Props
+
+### accept
+
+A string passed to `FormFileUpload` that tells the browser which file types can be upload to the upload window the browser use e.g: `image/*,video/*`.
+More information about this string is available in https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/file#Unique_file_type_specifiers.
+This property is similar to the `allowedTypes` property. The difference is the format and the fact that this property affects the behavior of `FormFileUpload` while `allowedTypes` affects the behavior `MediaUpload`.
+
+- Type: `String`
+- Required: No
+
+### addToGallery
+
+If true, and if  `gallery === true` the gallery media modal opens directly in the media library where the user can add additional images. When uploading/selecting files on the placeholder, the placeholder appends the files to the existing files list.
+If false the gallery media modal opens in the edit mode where the user can edit existing images, by reordering them, remove them, or change their attributes. When uploading/selecting files on the placeholder the files replace the existing files list.
+
+- Type: `Boolean`
+- Required: No
+- Default: `false`
+
+### allowedTypes
+
+Array with the types of the media to upload/select from the media library.
+Each type is a string that can contain the general mime type e.g: `image`, `audio`, `text`,
+or the complete mime type e.g: `audio/mpeg`, `image/gif`.
+If allowedTypes is unset all mime types should be allowed.
+This property is similar to the `accept` property. The difference is the format and the fact that this property affects the behavior of `MediaUpload` while `accept` affects the behavior `FormFileUpload`.
+
+- Type: `Array`
+- Required: No
+
+### className
+
+Class name added to the placeholder.
+
+- Type: `String`
+- Required: No
+
+### isAppender
+
+If true, the property changes the look of the placeholder to be adequate to scenarios where new files are added to an already existing set of files, e.g., adding files to a gallery.
+If false the default placeholder style is used.
+
+- Type: `Boolean`
+- Required: No
+- Default: `false`
+
+### labels
+
+An object that can contain a `title` and `instructions` properties. These properties are passed to the placeholder component as `label` and `instructions` respectively.
+
+- Type: `Object`
+- Required: No
+
+
+### multiple
+
+Whether to allow multiple selection of files or not.
+
+- Type: `Boolean`
+- Required: No
+- Default: `false`
+
+### onError
+
+Callback called when an upload error happens.
+
+- Type: `Function`
+- Required: No
+
+### onSelect
+
+Callback called when the files are selected/uploaded.
+The call back receives an array with the new files. Each element of the collection is an object containing the media properties of the file e.g.: `url`, `id`,...
+
+- Type: `Function`
+- Required: Yes
+
+### value
+
+Media ID (or media IDs if multiple is true) to be selected by default when opening the media library.
+
+- Type: `Number|Array`
+- Required: No
+
+
 ## Extend
 
 It includes a `wp.hooks` filter `editor.MediaPlaceholder` that enables developers to replace or extend it.