add perm installation guide

docs/installPermUbuntu18

+sudo apt install flex bison libxslt-dev libxml2-dev libz-dev gcc make clang
+git clone https://github.com/IITDBGroup/perm.git
+cd perm
+./configure --with-libxml --with-libxslt --prefix=INSTALLDIR -CFLAGS='-O1' -CC=clang
+make
+make install
+mkdir clusterdir
+
+vim ~/.bashrc
+add export PATH="/home/joker/permdir/bin:$PATH" to ~/.bashrc
+quit vim
+
+sudo chmod 777 -R /usr/local
+initdb -D /usr/local/pgsql/data
+postgres -D /usr/local/pgsql/data
+#open a new tab
+createdb test
+psql test
+CREATE LANGUAGE plpgsql;
+\i /pathToPermCode/contrib/xml2/pgxml.sql
+