Add doc to build on gentoo with overlay

doc/building-on-gentoo.md

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+# Gentoo building instructions
+
+## Install
+
+1. sudo layman -o https://raw.github.com/ddorian1/gentoo-twister-overlay/master/gentoo-twister-overlay.xml -a twister
+1. sudo emerge -av twister
+
+## Configuration & web gui
+
+1. mkdir ~/.twister
+1. echo -e "rpcuser=user\nrpcpassword=pwd" > ~/.twister/twister.conf
+1. chmod 600 ~/.twister/twister.conf
+1. git clone https://github.com/miguelfreitas/twister-html.git ~/.twister/html
+
+## Start
+
+1. twisterd -rpcuser=user -rpcpassword=pwd
+1. Open http://127.0.0.1:28332/index.html and use the user/pwd credentials
+1. Create your account !