Create Well-Known-Specification.md (GoogleChrome#121)

* Create Well-Known-Specification.md

Well-Known-Specification.md

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+# The "related-website-set.json" Well-Known Resource Identifier
+
+This page describes the use and content of URIs with the path
+`/.well-known/related-website-set.json`.
+## Use
+
+A site may demonstrate intentional opt-in to participation in a Related Website
+Set (RWS) by hosting a specific JSON file at the URI with a path of
+`/.well-known/related-website-set.json`.
+## Content
+
+The `/.well-known/related-website-set.json` file for the set primary of an RWS
+must follow the schema specified below:
+```json
+{
+  "type": "object",
+  "properties": {
+    "ccTLDs": {
+      "type": "object",
+      "additionalProperties": {
+        "type": "array",
+        "items": {
+          "type": "string"
+        }
+      }
+    },
+    "primary": { "type": "string" },
+    "associatedSites": {
+      "type": "array",
+      "items": {
+        "type": "string"
+      }
+    },
+    "serviceSites": {
+      "type": "array",
+      "items": {
+        "type": "string"
+      }
+    },
+    "rationaleBySite": {
+      "type": "object",
+      "additionalProperties": {
+        "type": "string"
+      }
+    }
+  },
+  "required": ["primary"],
+  "dependentRequired": {
+    "associatedSites": ["An explanation of how you clearly present the 
+    affiliation across domains to users and why users would expect your 
+    domains to be affiliated"],
+    "serviceSites": ["An explanation of how each domain in this subset 
+    supports functionality or security needs."]
+  }
+}
+```
+The `/.well-known/related-website-set.json` file for non-primary members of an
+RWS must follow the schema specified below:
+```json
+{
+  "type": "object",
+  "properties": {
+    "primary": { "type": "string" }
+  },
+  "required": ["primary"]
+}
+```