replace deprecated and removed set-env command

mvdan · mvdan · commit d89d6943c199 · 2020-11-16T09:08:57.000Z
See https://github.blog/changelog/2020-11-09-github-actions-removing-set-env-and-add-path-commands-on-november-16/. Fixes #11.
diff --git a/README.md b/README.md
@@ -60,14 +60,16 @@ jobs:
 
 #### How do I set environment variables at run-time?
 
-You can use [workflow commands](https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions)
-to set environment variables, add an element to `$PATH`, and more. For example:
+You can use [workflow commands](https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files)
+to set environment variables or add an element to `$PATH`. For example:
 
 ```yaml
-echo "::set-env name=CGO_ENABLED::0"
-echo "::add-path::${HOME}/goroot/bin"
+echo "CGO_ENABLED=0" >> $GITHUB_ENV
+echo "${HOME}/goroot/bin" >> $GITHUB_PATH
 ```
 
+Note that these take effect for future steps in the job.
+
 #### How do I set up caching between builds?
 
 Use [actions/cache](https://github.com/actions/cache). For example, to cache
