Update README.md: add instructions for SSH keys

README.md

 SSH and GPG public keys for Tétras Libre collaborators
 
+[[_TOC_]]
+
+# SSH
+SSH is used for Git access, among other things.
+## Creating SSH key
+```sh
+ssh-keygen -t ed25519 -f ~/.ssh/<your-name>_ed25519
+```
+If necessary, you can use the traditional RSA key type, instead of Ed25519, just use `-t rsa` instead and `<your-name>_rsa` for the key name.
+
+From the [Arch Linux wiki page](https://wiki.archlinux.org/title/SSH_keys#Generating_an_SSH_key_pair):
+> You can also add an optional comment to the public key with the `-C` switch, to more easily identify it in places such as `~/.ssh/known_hosts` [...]. For example:
+```sh
+-C "$(whoami)@$(uname -n)-$(date -I)"
+```
+> will add a comment saying which user created the key on which machine and when (like `user@machine-2023-08-14`) 
+
+## Adding key to the repo
+Simply copy your key from `~/.ssh/` to the repo's `SSH` folder.  
+**Be careful to add the `.pub` key and not your private key!**
+
+
 # GPG
 GPG is used for [pass](https://gitlab.tetras-libre.fr/tetras-libre-admin/pass)
 
-## Creating gpg key
-`gpg --generate-key`
-follow dialog
+## Creating GPG key
+```sh
+gpg --generate-key
+```
+and follow dialog.
 
 ## Add key of Tétras Libre collaborators
 To import a gpg key, use `gpg --import <key file>`. To import all keys, go to GPG folder and use `gpg --import *`