Add link to \openout primitive docs

services/clsi/README.md

@@ -35,7 +35,7 @@ The CLSI can be configured through the following environment variables:
 * `TEXLIVE_IMAGE` - The TeX Live Docker image to use for sibling containers, e.g. `gcr.io/overleaf-ops/texlive-full:2017.1`
 * `TEX_LIVE_IMAGE_NAME_OVERRIDE` - The name of the registry for the Docker image e.g. `gcr.io/overleaf-ops`
 * `TEXLIVE_IMAGE_USER` - When using sibling containers, the user to run as in the TeX Live image. Defaults to `tex`
-* `TEXLIVE_OPENOUT_ANY` - Sets the `openout_any` environment variable for TeX Live
+* `TEXLIVE_OPENOUT_ANY` - Sets the `openout_any` environment variable for TeX Live (see the `\openout` primitive [documentation](http://tug.org/texinfohtml/web2c.html#tex-invocation))
 
 Further environment variables configure the [metrics module](https://github.com/overleaf/metrics-module)