remove placeholder alt text from example (#78)

+ added alt text guidance

README.md

@@ -42,15 +42,15 @@ Hotgrid has a dynamic layout system. If you have 5 items but set the columns to
 
 >>**src** (string): File name (including path) of the image. Path should be relative to the *src* folder (e.g., *course/en/images/c-15.png*). Three src values are set to display: default, hover and visited states - **src** (default), **srcHover** and **srcVisited** - note that if you are handling the hover and visited states via CSS you don't need to set **srcHover** and **srcVisited**
 
->>**alt** (string): This text becomes the image’s `alt` attribute.
+>>**alt** (string): The alternative text for the item image. Assign [alt text](https://github.com/adaptlearning/adapt_framework/wiki/Providing-good-alt-text) to images that convey course content only. By default, the item is labelled by the **title**, if set, otherwise a generic 'Item 1, 2, 3 etc'. Followed by the alternative text, if set.
 
 >>**title** (string): This is optional text which is displayed under the grid item image.
 
 >**\_itemGraphic** (string): This is the image for a grid item pop-up. This also contains values **src**, **alt**, and **attribution**. You only need to set this if you want to display a different image in the popup.
 
 >>**src** (string): File name (including path) of the image. Path should be relative to the *src* folder (e.g., *course/en/images/origami-menu-two.jpg*).
 
->>**alt** (string): This text becomes the image’s `alt` attribute.   
+>>**alt** (string): The alternative text for the item popup image. Assign [alt text](https://github.com/adaptlearning/adapt_framework/wiki/Providing-good-alt-text) to images that convey course content only.
 
 >>**attribution** (string): Optional text to be displayed as an [attribution](https://wiki.creativecommons.org/Best_practices_for_attribution). By default it is displayed below the image. Adjust positioning by modifying CSS. Text can contain HTML tags, e.g., `Copyright © 2015 by <b>Lukasz 'Severiaan' Grela</b>`.
 

example.json

@@ -25,12 +25,12 @@
                     "src": "course/en/images/example.jpg",
                     "srcHover": "",
                     "srcVisited": "",
-                    "alt": "Placeholder graphic",
+                    "alt": "",
                     "title": "Item Title 1"
                 },
                 "_itemGraphic": {
                     "src": "course/en/images/example.jpg",
-                    "alt": "Placeholder graphic",
+                    "alt": "",
                     "attribution": "Copyright © 2019"
                 }
             },

properties.schema

@@ -154,9 +154,10 @@
                 "type": "string",
                 "required": false,
                 "default": "",
-                "title": "Item alt text",
+                "title": "Alternative text",
                 "inputType": "Text",
                 "validators": [],
+                "help": "A description of the image; required when it has meaning that must be conveyed to the learner. For 'decorative' images, leave this blank. By default, the item is labelled by the 'title', if set, otherwise a generic 'Item 1, 2, 3 etc'. Followed by the alternative text.",
                 "translatable": true
               },
               "title": {
@@ -188,9 +189,10 @@
                 "type": "string",
                 "required": false,
                 "default": "",
-                "title": "Item alt text",
+                "title": "Alternative text",
                 "inputType": "Text",
                 "validators": [],
+                "help": "A description of the image; required when it has meaning that must be conveyed to the learner. For 'decorative' images, leave this blank.",
                 "translatable": true
               },
               "attribution": {