Update migration v4 to v5 guide to include env variable changes

[JJPell]

This isn't mentioned in the migration guide but it would be useful to know that when migrating from v4 to v5 you need to change the format of the environment variables passed to your config from the following:

webpack --env.FOO foo --env.BAR --env.BAZ baz

To this format:

webpack --env FOO=foo BAR BAZ=baz

Otherwise you get the following error:

Error: Unknown option '--env.foo'

[yadunandanbhat]

i would like to work on this, but I might need some help!

[chenxsan]

It's more like a breaking change between webpack-cli v3 and v4 as per #4051. Not in webpack itself.