hyphen: add Russian dictionary, update readmeFileName values

For some reason README file for Russian has different naming upstream,
therefore `readmeFileName` for all languages was changed to accommodate
both naming schemes.

pkgs/development/libraries/hyphen/dictionaries.nix

···
         install -m644 "hyph_${dictFileName}.dic" "$out/share/hyphen"
         # docs
         install -dm755 "$out/share/doc/"
-        install -m644 "README_hyph_${readmeFileName}.txt" "$out/share/doc/${pname}.txt"
+        install -m644 "README_${readmeFileName}.txt" "$out/share/doc/${pname}.txt"
         runHook postInstall
       '';
     };
···
     shortName = "de-de";
     shortDescription = "German (Germany)";
     dictFileName = "de_DE";
-    readmeFileName = "de";
+    readmeFileName = "hyph_de";
   };
 
   de_AT = de-at;
···
     shortName = "de-at";
     shortDescription = "German (Austria)";
     dictFileName = "de_AT";
-    readmeFileName = "de";
+    readmeFileName = "hyph_de";
   };
 
   de_CH = de-ch;
···
     shortName = "de-ch";
     shortDescription = "German (Switzerland)";
     dictFileName = "de_CH";
-    readmeFileName = "de";
+    readmeFileName = "hyph_de";
+  };
+
+  # RUSSIAN
+
+  ru_RU = ru-ru;
+  ru-ru = mkDictFromLibreofficeGit {
+    subdir = "ru_RU";
+    shortName = "ru-ru";
+    shortDescription = "Russian (Russia)";
+    dictFileName = "ru_RU";
+    readmeFileName = "ru_RU";
   };
 }