Update documentation to clarify the process

I've finally managed to build the release of KOreader for Android, but I've run into some issues.
I've updated the manual to reflect the solutions I've found.
The issues I've found are:
1. The version of the needed Android SDK platform isn't described anywhere;
2. xutils-dev is missing from the prerequisites, while it provides the `makedepend` utility used by the openssl build system;
3. Finally, for the Android build it is necessary to install JDK version 8.

README.md

@@ -78,7 +78,7 @@ Users of Debian and Ubuntu can install the required packages using:
 ```
 sudo apt-get install build-essential git patch wget unzip \
 gettext autoconf cmake libtool nasm luarocks \
-libssl-dev libffi-dev libsdl2-dev libc6-dev-i386 linux-libc-dev:i386 zlib1g:i386
+libssl-dev libffi-dev libsdl2-dev libc6-dev-i386 xutils-dev linux-libc-dev:i386 zlib1g:i386
 ```
 
 Cross compile toolchains are available for Ubuntu users through these commands:
@@ -101,10 +101,21 @@ Mac OSX users may need to install these tools:
 brew install nasm binutils libtool autoconf automake sdl2
 ```
 
-A recent version of Android SDK/NDK and `ant` are needed in order to build
-KOReader for Android devices.
+A recent version of Android SDK (including platform support for API version 19)/NDK, `ant` and `openjdk-8-jdk` are needed
+in order to build KOReader for Android devices.
+
+Users of Debian first need to configure the `backports` repository:
+```
+sudo echo "deb http://ftp.debian.org/debian jessie-backports main" > /etc/apt/sources.list.d/backports.list
+sudo apt-get update
+```
+For both Ubuntu and Debian, install the packages:
+```
+sudo apt-get install ant openjdk-8-jdk
+```
+Users on Debian finally need to remove JRE version 7:
 ```
-sudo apt-get install ant
+sudo apt-get remove openjdk-7-jre-headless
 ```
 
 In order to build KOReader package for Ubuntu Touch, the `click` package management