Update Build instructions

technical/Build-instructions.md

@@ -109,11 +109,17 @@ On Android
 
 + Please make sure Android SDK and NDK are installed, and their paths are properly set. If you need
 more information, please visit <https://github.com/savoirfairelinux/ring-client-android>
++ These instructions are for Ubuntu, check equivalent if you use a different distribution
 
 #### Prepare environment
 
--   Install Java JDK 7 or 8:
+-   Install Java JDK 7 or 8 (Oracle or OpenJDK)
-    <http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html>
+
+<http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html>
+
+```bash
+sudo apt install openjdk
+```
 
 <!-- -->
 
@@ -127,8 +133,7 @@ more information, please visit <https://github.com/savoirfairelinux/ring-client-
 
 <!-- -->
 
--   Install required tools (on Ubuntu, check equivalent if you use a
+-   Install required tools
-    different distro)
 
 <!-- -->
 
@@ -202,6 +207,15 @@ cd ring-project
 
 -   `sudo` will use root's bash profile. And you don't need it.
 
+<!-- -->
+
+-   If build fails, you can try to clean contribs with:
+
+```bash
+cd ring-project/daemon && git clean -xdf
+```
+Note that the build will be longer.
+
 How to Build LibRing (or Daemon)
 ===========================================================