Adding JSON viewer explainer

Author: codepo8

explainers/JSONViewer/colour-coded.png

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+# DevTools: In-browser JSON viewer
+
+Authors:
+ - *[Chris Heilmann](https://twitter.com/codepo8)*, PM, Microsoft Edge
+
+## Status of this feature
+
+This is a proposal for a new feature that is not yet in development.
+
+## Proposal
+
+Currently when you navigate to a JSON file in Microsoft Edge, you get a basic text display of the document.
+
+![JSON displayed as plain text in the browser](current-json-display.png)
+
+When navigating to an HTML document, and selecting "view source" you get a colour-coded display with line numbers instead.
+
+![View source showing HTML in a more detailed fashion than just a text display](view-source.png)
+
+We propose to replace tha basic JSON view with a colour-coded, collapsible and expandable one.
+
+![JSON in browser as a colour-coded, collapsible tree](colour-coded.png)
+
+We are currently investigating the overall browser performance impact of this, which is why the first implementation will be basic.
+
+### Goals 
+
+As JSON files are data structures and users want to drill down into the data and analyse it in detail. This is a first step in this direction. In subsequent iterations we want to add more functionality like displaying header data and a raw view, saving and copying data and filtering the information.
+
+### Usage 
+
+This functionality will trigger automatically when the browser navigates to a JSON file on the web or the user opens a local one.
+
+## Feedback
+Have feedback? We want to hear it! *[Issue #123](link to issue announcing explainer, which will need to be added after first commit)* tracks community feedback on this explainer.