Add docs

docs/objects/Flexbox/setAlignItems.md

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+## setJustifyContent
+
+### Description
+
+Determines how the children are aligned along the off axis
+
+### Parameters
+
+1. `string` One of ("flex-start", "flex-end", "center", "space-between", "space-around") - default is flex-start. Works the same as the property of the same name in [CSS flexboxes](https://css-tricks.com/snippets/css/a-guide-to-flexbox/#aa-flexbox-properties)
+
+### Returns
+
+1. `object` The object in use
+
+### Usage
+
+* Creates a default flexbox and sets the align items to space-between.
+
+```lua
+local main = basalt.createFrame()
+local flexbox = mainFrame:addFlexbox()
+      :setAlignItems("space-between")
+```