add CI=true note

Author: t-bltg

README.md

@@ -9,7 +9,8 @@ The new documentation files will be pushed to the [`gh-pages`](https://github.co
 
 ## Building the docs
 
-With proper dependencies installed, run `GKSwstype=nul julia --project=docs/ docs/make.jl`.
+With proper dependencies installed, run `GKSwstype=nul CI=true julia --project=docs docs/make.jl`.
+Setting the environment variable `CI=true` is optional, but this will be closer to the remote docs building process.
 
 ## Contributing demos