Adds arxiv plugin to the plugins directory in docs

docs/plugins/directory.md

@@ -90,6 +90,16 @@ The following plugins add new {ref}`tools <tools>` that can be used by models:
 - **[llm-cluster](https://github.com/simonw/llm-cluster)** adds a `llm cluster` command for calculating clusters for a collection of embeddings. Calculated clusters can then be passed to a Large Language Model to generate a summary description.
 - **[llm-jq](https://github.com/simonw/llm-jq)** lets you pipe in JSON data and a prompt describing a `jq` program, then executes the generated program against the JSON.
 
+## Fragments and template loaders
+
+{ref}`LLM 0.24 <v0_24>` introduced support for plugins that define `-f prefix:value` or `-t prefix:value` custom loaders for fragments and templates.
+
+- **[llm-templates-github](https://github.com/simonw/llm-templates-github)** supports loading templates shared on GitHub, e.g. `llm -t gh:simonw/pelican-svg`.
+- **[llm-templates-fabric](https://github.com/simonw/llm-templates-fabric)** provides access to the [Fabric](https://github.com/danielmiessler/fabric) collection of prompts: `cat setup.py | llm -t fabric:explain_code`.
+- **[llm-fragments-github](https://github.com/simonw/llm-fragments-github)** can load entire GitHub repositories in a single operation: `llm -f github:simonw/files-to-prompt 'explain this code'`.
+- **[llm-hacker-news](https://github.com/simonw/llm-hacker-news)** imports conversations from Hacker News as fragments: `llm -f hn:43615912 'summary with illustrative direct quotes'`.
+- **[llm-arxiv](https://github.com/agustif/llm-arxiv)** provides a `-f arxiv:2504.17544` loader that can load and parse arxiv PDF research papers as fragments.
+
 (plugin-directory-fun)=
 ## Just for fun