Android build: add comments on filesystem layout

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

packaging/android/build.sh

@@ -1,4 +1,24 @@
 #!/bin/bash
+#
+# Filesystem layout considerations...
+# for this explanation I assume that your Subsurface sources are in
+# ~/src/subsurface
+#
+# You need to have a version of Qt that contains the Android bits
+# installed. You should be able to find the correct installer for
+# Linux or Mac here:
+# http://download.qt.io/official_releases/qt/5.7/
+# make sure you pick one with 'android' in its name.
+#
+# Install this wherever you want - and then have a link named ~/src/Qt that
+# points to it.
+# So let's assume that you are installing the package above in ~/Qt5.7.1
+# (which I think is the default location), then simply do
+# cd ~/src
+# ln -s ~/Qt5.7.1 Qt
+#
+# you also need to have the current Android SDK and NDK installed under ~/src
+#
 set -e
 PLATFORM=$(uname)
 # (trick to get the absolute path, either if we're called with a