Translate english plurals

Turns out that for Qt to properly handle plurals in English a
translation needs to be provided, otherwise the user is left with
messages such as "Building: 2 shader(s)"

Plurals for other all other languages are handled on transifex.
I wrote the README.md to just refer to it as a translation
collaboration site just in case we ever switch.

These translations being out of date won't pose any technical problems
so I believe it is fine to handle them manually on a "best effort"
basis.

The files are generated into the source directory so that the
relative filenames are correct. The generated file is added to
.gitignore

src/yuzu/configuration/configure_ui.cpp

@@ -219,6 +219,7 @@ void ConfigureUi::InitializeLanguageComboBox() {
     for (const auto& lang : languages) {
         if (QString::fromLatin1(lang.id) == QStringLiteral("en")) {
             ui->language_combobox->addItem(lang.name, QStringLiteral("en"));
+            language_files.removeOne(QStringLiteral("en.qm"));
             continue;
         }
         for (int i = 0; i < language_files.size(); ++i) {