[misc] Update documentation to build aegisub as an AppImage

Also add the --enable-appimage that will download the appimage creation tool

.gitignore

 *.App
+*.AppImage
 *.[oadi]
 *.ass
 *.avi

README.md

@@ -56,12 +56,15 @@ On debian buster, the packages are the following:
 - libboost-thread-dev
 - astyle
 
+### Build the binary
+
 Once all the dependencies are installed, run:
 
 ```bash
 autoreconf
 ./configure --enable-debug --with-libpulse
 make -j$(nproc)
+make install
 ```
 
 You may also consider the following rules:
@@ -71,6 +74,26 @@ You may also consider the following rules:
 - `make tags` to build the ctags and cscope databases
 - `make style` to format your code with the right style
 
+### Build an appimage
+
+First download the appimagetool binary and save it to the [tools](tools)
+folder. If you don't do it manually, it will be done for you at some point by
+the build scripts.
+
+```bash
+wget https://github.com/probonopd/AppImageKit/releases/download/continuous/appimagetool-x86_64.AppImage \
+    -O tools/appimagetool.AppImage
+```
+
+Then, build normally aegisub, but with the appimage options:
+
+```bash
+autoreconf
+./configure --enable-debug --with-libpulse --enable-appimage
+make -j$(nproc)
+make appimage
+```
+
 ### Windows
 
 Prerequisites:

configure.ac

@@ -182,6 +182,26 @@ AC_CHECK_HEADERS([sys/time.h])
 AC_ARG_ENABLE(sanity-checks,
               AS_HELP_STRING([--disable-sanity-checks],[Skip verifying that found libraries work.]))
 
+#############################
+# Build with appimage support
+#############################
+AC_ARG_ENABLE([appimage],
+              [AS_HELP_STRING([--enable-appimage], [Build Aegisub as an AppImage. Defaults to no])],
+              [AEGISUB_APPIMAGE_ENABLED=$enableval],
+              [AEGISUB_APPIMAGE_ENABLED=no])
+AS_IF([test "x$AEGISUB_APPIMAGE_ENABLED" = "xyes"], [
+       AC_MSG_CHECKING([Downloading AppImage creation tool])
+       wget https://github.com/probonopd/AppImageKit/releases/download/continuous/appimagetool-x86_64.AppImage \
+           -O tools/appimagetool.AppImage -o config.wget.log
+       AS_IF([test $? -eq 0 ], [
+           AC_MSG_RESULT([done])
+        ], [
+           AC_MSG_FAILURE([failed])
+        ])
+], [
+    AC_MSG_NOTICE([Don't build with AppImage support])
+])
+
 ##############################
 # Program Support and Features
 ##############################