Make "unofficial" build options modular and dynamic (#568)

* Make "unofficial" build options modular and dynamic

* Specify codeblock type in README.md

Gruntfile.coffee

@@ -138,19 +138,11 @@ module.exports = (grunt) ->
             afterRemove: 'bin/deb/after-remove.tpl'
             category: 'Network'
             depends: ['libappindicator1', 'libasound2', 'libgconf-2-4', 'libnotify-bin', 'libnss3', 'libxss1']
-      linux_ia32:
+      linux_other:
         options:
-          arch: 'ia32'
-          productName: 'wire-desktop'
-          targets: ['dir']
-          linux:
-            fpm: ['--name', 'wire-desktop']
-            executableName: 'wire-desktop'
-      linux_x64:
-        options:
-          arch: 'x64'
+          arch: "#{grunt.option('arch') || process.arch}"
           productName: 'wire-desktop'
-          targets: ['dir']
+          targets: ["#{grunt.option('target') || 'dir'}"]
           linux:
             fpm: ['--name', 'wire-desktop']
             executableName: 'wire-desktop'

README.md

@@ -61,6 +61,17 @@ npm run build:win
 npm run build:linux
 ```
 
+### Other Linux targets or architectures
+
+If you would like to build for another Linux target or architecture, run the following commands:
+
+```bash
+grunt 'clean:linux' 'update-keys' 'release-prod'
+grunt --arch=<arch> --target=<target> 'electronbuilder:linux_other'
+```
+
+Replace `<arch>` and `<target>` with your desired architecture (e.g. `"ia32"`) and target (e.g. `"rpm"`). Have a look at the [documentation for `electron-builder`](https://github.com/electron-userland/electron-builder/wiki/Options) for the available options. Note that we cannot offer support for uncommon architectures or targets.
+
 ### Troubleshooting
 
 If you are having troubles building Wire for Desktop, then [our troubleshooting page](https://github.com/wireapp/wire-desktop/wiki/Troubleshooting) might be of help.