add readme

Author: b5

extism/readme.md

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+# iroh + exsitm
+
+This is an example of running iroh thorugh [exitsm](https://extism.org/) as a set of [host functions](https://extism.org/docs/concepts/host-functions). It's written entirely in rust, but the purpose is to show how one would build an API that any plugin could use to gain access to a plugin-host-provided iroh instance.
+
+## Requirements
+
+You'll need the latest version of the rust toolchain, and the `wasm32-unknown-unknown` target. Install it with:
+
+```
+rustup target add wasm32-unknown-unknown
+```
+
+## Running the Example
+
+There are three 
+
+1. Build the WASM plugin
+2. Get a blob ticket
+3. run the host
+
+### 1. Build the WASM plugin
+
+`cd` to `extism/plugin` and run `cargo build --target wasm32-unknown-unknown`. If you get an error, check the requirements section above. This will generate a WASM plugin that actually calls the blob download host function.
+
+### 2. Get a blob ticket
+
+This example needs a `Raw` blob ticket, which you can get from `iroh blob add` on a raw file, or just use this one, which should be hosted on iroh.network:
+```
+blobabk62aofuwwwu5zb5ocvzj5v3rtqt6siglyuhoxhqtu4fxravvoteajcnb2hi4dthixs65ltmuys2mjomrsxe4bonfzg62bonzsxi53pojvs4lydaac2cyt22erablaraaa5ciqbfiaqj7ya6cbpuaaaaaaaaaaaahjceagucztuhgez4qucv2733xphmgpc2nkgj54od2vuygn6sz4zzxo6ce
+```
+(it's a text file that says hello)
+
+### 3. Run the host
+
+The host code will actually call the example. `cd` to `./extism/host` and run
+
+```
+cargo run <TICKET>
+```
+
+pasting in the ticket you'd like to fetch. It'll fetch the bytes & print it out, proving this host can provide iroh to extism plugins!
+
+
+## Future work
+
+What would be truly neat would be to reverse the direction, and write a plugin that brings iroh to a host. For that we'd need to be able to compile some/all of iroh to WASM, which we're [working on](https://github.com/n0-computer/iroh/issues/1803).