Merge pull request LedgerHQ#12 from cfranceschi-ledger/dev-portal-dis…

…cord

Paragraph about Dev Portal and Discord

README.md

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+# Nano X Secure SDK
+
+## Are you developing a Nano X application?
+
+If so, you must use this SDK. 
+
+For a smooth and quick integration:
+- See the developers’ documentation on the [Developer Portal](https://developers.ledger.com/) and 
+- Go on Discord to chat with developper support and the developper community. See you there! If you are new to Ledger OP3N Discord server [click here](https://discord.gg/Ledger), otherwise directly join [the Nano App channel](https://discord.com/channels/885256081289379850/907623554542080070).
+