Add TOC to README

README.md

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 - **7 continents** (7 cards, maps only)
 - for a total of **319 unique notes**, **1095 cards**, **260 flags** and **318 maps**.
 
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+- [Features](#features)
+  - [Note templates](#note-templates)
+  - [Supplementary information](#supplementary-information)
+  - [Translations](#translations)
+  - [Extended deck](#extended-deck)
+  - [Custom study](#custom-study)
+- [Getting started](#getting-started)
+  - [Other languages and versions](#other-languages-and-versions)
+  - [Stay in touch](#stay-in-touch)
+- [Upgrading](#upgrading)
+  - [Minor version](#minor-version)
+  - [Major version](#major-version)
+- [Background](#background)
+- [Deck structure](#deck-structure)
+
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 ## Features