Update README to include gettext installation

README.md

@@ -28,6 +28,7 @@ As [Weblate does not yet support Markdown](https://github.com/WeblateOrg/weblate
 - All `.po` translation updates should be followed by regenerating the localized Markdown files used by Hugo (in locale-specific content directories defined in `config.toml`) by running `npm run po4a`.
 - Adding/removing FAQ entries or locales should be followed by first regenerating the Po4a config file using `po/genconf.sh > po/po4a.conf`, and then updating the `.po` files with `npm run po4a`.
 - Adding a new locale should at some point be followed by telling Privacy Badger to [use the localized FAQ link](https://github.com/EFForg/privacybadger/blob/a8bd923d973db5b46da1b48930232cf4f114e87c/src/lib/i18n.js#L27) when the user's browser is in that locale.
+- If `npm run po4a` fails because `msgmerge` is not found, install the `gettext` package.
 
 To install Po4a, check out [its repository](https://github.com/mquinson/po4a), switch to the `v0.66` tag, and then create a helper script to [run `po4a` from the checkout](https://github.com/mquinson/po4a#use-without-installation). For example, create `~/.local/bin/po4a`, set the file as executable, and save the following text there, replacing `/PATH/TO/CHECKOUT/` with your actual checkout path: