add doc for linux

docs/linux.md

@@ -0,0 +1,56 @@
+# How to receive `obsidian://` in Linux
+
+## Background
+
+For example, when we are authorizing OneDrive, we have to jump back to Obsidian automatically using `obsidian://`.
+
+## Short Desc From Official Obsidian Doc
+
+Official doc has some explanation:
+
+<https://help.obsidian.md/Extending+Obsidian/Obsidian+URI#Register+Obsidian+URI>
+
+# Long Desc
+
+Assuming the username is `somebody`, and the `.AppImage` file is downloaded to `~/Desktop`.
+
+1. Download and **extract** the app image file in terminal
+
+   ```bash
+   cd /home/somebody/Desktop
+   chmod +x Obsidian-x.y.z.AppImage
+   ./Obsidian-x.y.z.AppImage --appimage-extract
+
+   # you should have the folder squashfs-root
+   # we want to rename it
+   mv squashfs-root Obsidian
+   ```
+
+2. Create a `.desktop` file
+
+   ```bash
+   # copy and paste the follow MULTI LINE command
+   # you might need to input your password because it requires root privilege
+   # remember to adjust the path
+   cat > ~/Desktop/obsidian.desktop <<EOF
+   [Desktop Entry]
+   Name=Obsidian
+   Comment=obsidian
+   Exec=/home/somebody/Desktop/Obsidian/obsidian %u
+   Keywords=obsidian
+   StartupNotify=true
+   Terminal=false
+   Type=Application
+   Icon=/home/somebody/Desktop/Obsidian/obsidian.png
+   MimeType=x-scheme-handler/obsidian;
+   EOF
+
+   # yeah we can check out the output
+   cat ~/Desktop/obsidian.desktop
+   ## [Desktop Entry]
+   ## ...
+   ```
+
+3. Right click the `obsidian.desktop` file on the Desktop, and click "Allow launching"
+
+4. Double click the `obsidian.desktop` file.