add nip-20: web comments.

20.md

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+NIP-20
+======
+
+Web Comments
+------------
+
+`draft` `optional` `author:fiatjaf`
+
+A special event with kind `34` is defined, meaning "web comment".
+
+Events of this kind should have at least one `r` tag with the value set to the normalized URL of the webpage they're commenting in.
+
+The content should be the plaintext of the comment.
+
+For example:
+
+```json
+{
+  "kind": 34,
+  "tags": [
+    ["r", "https://random.blog/article"]
+  ],
+  "content": "I didn't like this article.",
+  ...other fields
+```
+
+URL Normalization
+-----------------
+
+Early prototypes have been using the following naïve URL normalization algorithm:
+
+```js
+export function normalizeURL(raw) {
+  let url = new URL(raw)
+  return (
+    url.origin
+      .replace('://m.', '://') // remove known 'mobile' subdomains
+      .replace('://mobile.', '://')
+      .replace('http://', 'https://') // default everything to https (maybe a terrible idea)
+      .replace( /:\d+/, port => (port === ':443' || port === ':80' ? '' : port)) + // remove 443 and 80 ports
+    url.pathname
+      .replace(/\/+/g, '/') // remove duplicated slashes in the middle of the path
+      .replace(/\/*$/, '') // remove slashes from the end of path
+
+    // notice that only origin ("https://random.blog") and pathname ("/article") are used, all the rest is thrown away
+  )
+}
+```
+
+This is open for improvement, of course.