Currently `revision.txt` only exists on Heroku, since it's generated
by the Heroku-only `post_compile` script, just prior to `yarn build`.
However this means:
* HTTP 404s of `revision.txt` are seen in the browser console when
developing locally, which gives the appearance of something being
broken, even though it's not.
* when we convert the wildcard `CopyPlugin` rule to an explicit list
of files to copy (in a later commit), it will cause errors when
building locally, since `CopyPlugin` expects all declared files
to exist.
Adding a placeholder file prevents both of the above.
Ed Morley