docs(en): write configuration pages

docs/en-US/configuration/configuration.md

@@ -1 +1,50 @@
 # Configuration
+
+termscp supports a number of user-defined parameters. termscp stores them in a
+TOML file and a few directories, but you never edit these files by hand:
+configuration is done entirely from the user interface.
+
+To enter the configuration, press `<CTRL+C>` from the termscp home.
+
+termscp requires these paths to be accessible:
+
+- `$HOME/.config/termscp/` on Linux/BSD
+- `$HOME/Library/Application Support/termscp` on macOS
+- `FOLDERID_RoamingAppData\termscp\` on Windows
+
+## Parameters
+
+The following parameters can be configured:
+
+- **Text Editor**: the text editor to use. By default termscp finds the default
+  editor for you; with this option you can force an editor to be used (e.g.
+  `vim`). GUI editors are also supported, unless they detach (`nohup`) from the
+  parent process.
+- **Default Protocol**: the default value for the file transfer protocol to be
+  used in termscp. It applies to the login page and to the address CLI argument.
+- **Show Hidden Files**: whether hidden files are displayed by default. You can
+  also toggle hidden files at runtime by pressing `A`.
+- **Check for updates**: if set to `yes`, termscp queries the GitHub API to
+  check whether a new version of termscp is available.
+- **Prompt when replacing existing files**: if set to `yes`, termscp prompts for
+  confirmation whenever a file transfer would replace an existing file on the
+  target host.
+- **Group Dirs**: whether directories are grouped in the file explorers. If
+  `Display first` is selected, directories are sorted with the configured method
+  but displayed before files; with `Display last` they are displayed after
+  files.
+- **Remote file formatter syntax**: syntax used to display file info for each
+  file in the remote explorer. See [File explorer format](explorer-format.md).
+- **Local file formatter syntax**: syntax used to display file info for each file
+  in the local explorer. See [File explorer format](explorer-format.md).
+- **Enable notifications**: if set to `Yes`, desktop notifications are displayed.
+  See [Notifications](notifications.md).
+- **Notifications: minimum transfer size**: if the transfer size is greater than
+  or equal to the specified value, transfer notifications are displayed. The
+  accepted format is `{UNSIGNED} B/KB/MB/GB/TB/PB`.
+- **SSH configuration path**: SSH configuration file to use when connecting to a
+  SCP/SFTP server. If left empty, no file is used. You can specify a path
+  starting with `~` to indicate the home directory (e.g. `~/.ssh/config`). The
+  attributes supported by termscp are listed at
+  [the ssh2-config exposed attributes](https://github.com/veeso/ssh2-config#exposed-attributes).
+  See also [SSH key storage](ssh-keys.md).