docs: split and improve

docs/usage.md

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+<!-- order: 25 -->
+
+# Usage
+
+## Table of Contents
+
+- [Sign in with GitHub](#signin-github)
+- [Accounts authentication](https://github.com/VSCodium/vscodium/blob/master/docs/accounts-authentication.md)
+- [How do I run VSCodium in portable mode?](#portable)
+- [How do I fix the default file manager?](#file-manager)
+- [How do I press and hold a key and have it repeat in VSCodium?](#press-and-hold)
+- [How do I open VSCodium from the terminal?](#terminal-support)
+  - [From Linux .tar.gz](#from-linux-targz)
+
+## <a id="signin-github"></a>Sign in with GitHub
+
+In VSCodium, `Sign in with GitHub` is using a Personal Access Token.<br />
+Follow the documentation https://docs.github.com/en/github/authenticating-to-github/creating-a-personal-access-token to create your token.<br />
+Select the scopes dependending on the extension which needs access to GitHub. (GitLens requires the `repo` scope.)
+
+### Linux
+
+If you are getting the error `Writing login information to the keychain failed with error 'The name org.freedesktop.secrets was not provided by any .service files'.`, you need to install the package `gnome-keyring`.
+
+## <a id="portable"></a>How do I run VSCodium in portable mode?
+You can follow the [Portable Mode instructions](https://code.visualstudio.com/docs/editor/portable) from the Visual Studio Code website.
+- **Windows** / **Linux** : the instructions can be followed as written.
+- **macOS** : portable mode is enabled by the existence of a specially named folder. For Visual Studio Code that folder name is `code-portable-data`. For VSCodium, that folder name is `codium-portable-data`. So to enable portable mode for VSCodium on Mac OS, follow the instructions outlined in the [link above](https://code.visualstudio.com/docs/editor/portable), but create a folder named `codium-portable-data` instead of `code-portable-data`.
+
+## <a id="file-manager"></a>How do I fix the default file manager (Linux)?
+
+In some cases, VSCodium becomes the file manager used to open directories (instead of apps like Dolphin or Nautilus).<br />
+It's due to that no application was defined as the default file manager and so the system is using the latest capable application.
+
+To set the default app, create the file `~/.config/mimeapps.list` with the content like:
+```
+[Default Applications]
+inode/directory=org.gnome.Nautilus.desktop;
+```
+
+You can find your regular file manager with the command:
+```
+> grep directory /usr/share/applications/mimeinfo.cache
+inode/directory=codium.desktop;org.gnome.Nautilus.desktop;
+```
+
+## <a id="press-and-hold"></a>How do I press and hold a key and have it repeat in VSCodium (Mac)?
+
+This is a common question for Visual Studio Code and the procedure is slightly different in VSCodium because the `defaults` path is different.
+
+```bash
+$ defaults write com.vscodium ApplePressAndHoldEnabled -bool false
+```
+
+## <a id="terminal-support"></a>How do I open VSCodium from the terminal?
+
+For macOS and Windows:
+- Go to the command palette (View | Command Palette...)
+- Choose `Shell command: Install 'codium' command in PATH`.
+
+![](https://user-images.githubusercontent.com/2707340/60140295-18338a00-9766-11e9-8fda-b525b6f15c13.png)
+
+This allows you to open files or directories in VSCodium directly from your terminal:
+
+```bash
+~/in-my-project $ codium . # open this directory
+~/in-my-project $ codium file.txt # open this file
+```
+
+Feel free to alias this command to something easier to type in your shell profile (e.g. `alias code=codium`).
+
+On Linux, when installed with a package manager, `codium` has been installed in your `PATH`.
+
+### <a id="from-linux-targz"></a>From Linux .tar.gz
+
+When the archive `VSCodium-linux-<arch>-<version>.tar.gz` is extracted, the main entry point for VSCodium is `./bin/codium`.