Update pr template

Author: bebraw

PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,4 @@
 1. [Read and sign the CLA](https://cla.js.foundation/webpack/webpack.js.org). This needs to be done only once. PRs that haven't signed it won't be accepted.
 2. Make sure your PR complies with [the writer's guide](https://webpack.js.org/writers-guide/).
 3. Read through the PR diff carefully as sometimes this can reveal issues. The work will be reviewed, but this can save some effort.
+4. Remove these instructions from your PR as they are for your eyes only.