initial commit for building AppImages for Linux x86_64

contrib/build-linux/appimage/README.md

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+AppImage binary for Electrum
+============================
+
+This assumes an Ubuntu host, but it should not be too hard to adapt to another
+similar system. The docker commands should be executed in the project's root
+folder.
+
+1. Install Docker
+
+    ```
+    $ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
+    $ sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
+    $ sudo apt-get update
+    $ sudo apt-get install -y docker-ce
+    ```
+
+2. Build image
+
+    ```
+    $ sudo docker build --no-cache -t electrum-appimage-builder-img contrib/build-linux/appimage
+    ```
+
+3. Build binary
+
+    ```
+    $ sudo docker run -it \
+        --name electrum-appimage-builder-cont \
+        -v $PWD:/opt/electrum \
+        --rm \
+        --workdir /opt/electrum/contrib/build-linux/appimage \
+        electrum-appimage-builder-img \
+        ./build.sh
+    ```
+
+4. The generated binary is in `./dist`.
+
+
+## FAQ
+
+### How can I see what is included in the AppImage?
+Execute the binary as follows: `./electrum*.AppImage --appimage-extract`