Create guestbook.md with usage instructions

Added a new guestbook feature for users to store and display messages.

wiki/source/guestbook.md

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+---
+title: Tilde Guestbook
+author: deepend
+category: software
+---
+
+A simple, shared guestbook hosted at **guestbook.tilde.club** that you can link to from your personal tilde page.
+
+## What it does
+
+* Stores messages (name, email, message) per tilde user.
+* Renders your guestbook at:
+  `https://guestbook.tilde.club/index.php?user=<your_username>&theme=<theme_or_default>`
+
+## Requirements
+
+* **Link must come from your tilde page**: the request’s referrer must start with
+  `https://tilde.club/~<your_username>/`
+  If this doesn’t match, the guestbook shows *“Access denied: Invalid referrer.”*
+
+## Quick start
+
+1. On your tilde page (e.g., `~/public_html/index.html`), add a link:
+
+   ```html
+   <a href="https://guestbook.tilde.club/index.php?user=<your_username>&theme=default">
+     Sign my guestbook
+   </a>
+   ```
+
+## Optional theme
+
+* Create a CSS file in your tilde home, e.g. `~/public_html/guestbook.css`.
+* Link to the guestbook with `&theme=guestbook` (omit `.css`).
+  The guestbook will load:
+  `https://tilde.club/~<your_username>/guestbook.css`
+* If you don’t set `theme`, it uses `https://guestbook.tilde.club/default.css`.
+
+## CSS customization (what you can style)
+
+When you pass a `theme`, **only your CSS is loaded** (the default stylesheet is not). Your CSS should style the page completely.
+
+### Stable HTML hooks (selectors you can rely on)
+
+* Page title: `h1` (has `align="center"`; override with CSS if desired).
+* Intro paragraph and form:
+
+  * `form`
+  * `input[type="text"]` for Name and Email
+  * `textarea` for Message
+  * `input[type="submit"]` for the button
+* Entries list:
+
+  * Container: `.entries`
+  * Each entry: `.entry`
+  * Entry header: `.entry h3`
+  * Email inside header: `.entry h3 span`
+  * Message text: `.entry p`
+
+## Embedding notes
+
+* Works fine when linked from inside frames/iframes **as long as** the referrer includes your full path (`/~<user>/...`). The meta tag above usually fixes “origin-only” referrers.
+* Do **not** use `rel="noreferrer"` on the link—this strips the referrer entirely.
+
+## Troubleshooting
+
+* **“Access denied: Invalid referrer.”**
+
+  * Make sure the link is on `https://tilde.club/~<your_username>/...` (not just the domain root).
+  * Check that browser extensions or privacy settings aren’t stripping referrers.
+  * The error page will display what referrer was received and the expected pattern.
+
+## Example links
+
+* Default styling:
+
+  ```
+  https://guestbook.tilde.club/index.php?user=deepend&theme=default
+  ```
+* Custom styling (with `~/public_html/guest.css` present):
+
+  ```
+  https://guestbook.tilde.club/index.php?user=deepend&theme=guest
+  ```