Introduce CODEOWNERS file (#4670)

Signed-off-by: Marcin Mikołajczak <me@m4sk.in>

CODEOWNERS

+# CODEOWNERS for tootsuite/mastodon
+
+# Translators
+# To add translator, copy these lines, replace `fr` with appropriate language code and replace `@żelipapą` with user's GitHub nickname preceded by `@` sign or e-mail address.
+# /app/javascript/mastodon/locales/fr.json @żelipapą
+# /app/views/user_mailer/*.fr.html.erb @żelipapą
+# /app/views/user_mailer/*.fr.text.erb @żelipapą
+# /config/locales/*.fr.yml @żelipapą
+# /config/locales/fr.yml @żelipapą
+
+/app/javascript/mastodon/locales/pl.json @m4sk1n
+/app/views/user_mailer/*.pl.html.erb @m4sk1n
+/app/views/user_mailer/*.pl.text.erb @m4sk1n
+/config/locales/*.pl.yml @m4sk1n
+/config/locales/pl.yml @m4sk1n