ICU-20088 index page for ICU GitHub Pages

Entry point to find the new home of the User Guide.
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+<!--
+© 2020 and later: Unicode, Inc. and others.
+License & terms of use: http://www.unicode.org/copyright.html
+-->
+
+# ICU Documentation
+
+[![ICU logo](https://github.com/unicode-org/icu-docs/raw/master/img/iculogo_64.png)](http://site.icu-project.org/)
+
+## ICU User Guide
+
+The [ICU User Guide](./userguide/) provides information on i18n topics for which ICU has services, and
+includes details that go beyond the C, C++, and Java API docs (and avoids some duplication between them).
+
+This is the new home of the User Guide (since 2020 August).
+
+## ICU team member pages
+
+Other documentation pages here are written by and for team members.