feat(mentionhelp): add README with cog functionality and installation instructions

README.md

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+# MentionHelp
+
+Cog for Red-DiscordBot that sends a help embed when someone mentions the bot directly.
+
+## What it does
+
+- Detects direct mention messages:
+  - `<@BOT_ID>`
+  - `<@!BOT_ID>`
+- Replies with an embed that tells users to run:
+  - `[prefix]help`
+- Uses bot/server locale to choose response language:
+  - `pl*` -> Polish
+  - anything else -> English
+- If locale lookup fails, it safely falls back to English.
+
+## Locale priority
+
+Locale is resolved using Red's guild-aware locale API, so server locale takes priority over global locale.  
+If no valid locale is available, English is used.
+
+## Installation
+
+1. Put this cog folder in your custom cogs path.
+2. In Discord:
+
+```text
+[p]load mentionhelp
+```
+
+## Notes
+
+- The cog only responds in guilds (not in DMs).
+- It ignores bot messages.