[Minor] Add instructions for update

diff --git a/contrib/publicsuffix/README.md b/contrib/publicsuffix/README.md
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+# Public suffixes list
+
+Update procedure:
+
+1. Download the list from the [official mirror](https://publicsuffix.org/list/public_suffix_list.dat)
+2. Proceed through `idn.pl` script
+
+1 liner: `curl https://publicsuffix.org/list/public_suffix_list.dat | idn.pl > effective_tld_names.dat`