add gradient guide

add_gradients.md

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+# Guide for adding gradient in gradients.json file
+
+### Color groups
+
+```
+['#259BE5', '#E5E9EC', '#7BCC9B', '#F9AFAD', '#FC96D3', '#8B56E9']
+```
+### JSON structure
+```
+"name": "Winter Neva",  // gradient name
+"favorite": false,  // default value
+"index": 1,  // gradient index in array ( check last index in json file )
+"deg": 120,  // gradient degree ( top - zero degree )
+"group": [ "#259BE5" ],  // check color groups, may contains more than one value
+"gradient": [  // color array, may contains more than one object
+    {
+        "color": "#a1c4fd",  // HEX
+        "pos": 0 // color position in gradient
+    },
+    {
+        "color": "#c2e9fb",
+        "pos": 100
+    }
+]
+```