Add perplexity extension readme (#17861)

Peter Tripp created

Release Notes:

- N/A

Change summary

extensions/perplexity/README.md | 41 +++++++++++++++++++++++++++++++++++
1 file changed, 41 insertions(+)

Detailed changes

extensions/perplexity/README.md 🔗

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+# Zed Perplexity Extension
+
+This example extension adds the `/perplexity` [slash command](https://zed.dev/docs/assistant/commands) to the Zed AI assistant.
+
+## Usage
+
+Open the AI Assistant panel (`cmd-r` or `ctrl-r`) and enter:
+
+```
+/perplexity What's the weather in Boulder, CO tomorrow evening?
+```
+
+## Development Setup
+
+1. Install the rust toolchain and clone the zed repo:
+```
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+
+mkdir -p ~/code
+cd ~/code
+git clone https://github.com/zed-industries/zed
+```
+
+2. Launch Zed and Open Zed Extensions (`cmd-shift-x` / `ctrl-shift-x`)
+3. Open Zed
+4. Open Zed Extensions (`cmd-shift-x` / `ctrl-shift-x`)
+5. Click "Install Dev Extension"
+6. Navigate to the "extensions/perplexity" folder inside the zed git repo.
+7. Ensure your `PERPLEXITY_API_KEY` environment variable is set (instructions below)
+```sh
+env |grep PERPLEXITY_API_KEY
+```
+8. Quit and relaunch Zed
+
+## PERPLEXITY_API_KEY
+
+This extension requires a Perplexity API key to be available via the `PERPLEXITY_API_KEY` environment variable.
+
+To onbtain a Perplexity.ai API token, login to your Perplexity.ai account and go [Settings->API](https://www.perplexity.ai/settings/api) and under "API Keys" click "Generate".  This will require you to have [Perplexity Pro](https://www.perplexity.ai/pro) or to buy API credits. By default the extension uses `llama-3.1-sonar-small-128k-online`, currently cheapest model available which is roughly half a penny per request + a penny per 50,000 tokens.  So most requests will cost less than $0.01 USD.
+
+Take your API key and add it to your environment by adding `export PERPLEXITY_API_KEY="pplx-0123456789abcdef..."` to your `~/.zshrc` or `~/.bashrc`. Reload close and reopen your terminal session. Check with `env |grep PERPLEXITY_API_KEY`.