Merge branch 'update_centos' into 'master'

Centos guide now follows GitLab official guide.

Closes #2 #3 #4

Author: axilleas

e-mail/configure_email.md

@@ -0,0 +1,38 @@
+## Centos - Sendmail
+
+    su -
+    yum -y install sendmail-cf
+    cd /etc/mail
+    vim /etc/mail/sendmail.mc
+
+Add a line with the smtp gateway hostname
+
+    define(`SMART_HOST', `smtp.example.com')dnl
+
+Then replace this line:
+
+    EXPOSED_USER(`root')dnl
+
+with:
+
+    dnl EXPOSED_USER(`root')dnl
+
+Now enable these settings with:
+
+    make
+    chkconfig sendmail on
+
+### Forwarding all emails
+
+Now we want all logging of the system to be forwarded to a central email address:
+
+    su -
+    echo [email protected] > /root/.forward
+    chown root /root/.forward
+    chmod 600 /root/.forward
+    restorecon /root/.forward
+
+    echo [email protected] > /home/git/.forward
+    chown git /home/git/.forward
+    chmod 600 /home/git/.forward
+    restorecon /home/git/.forward