Add Ruby upgrade instructions

doc/update/ruby.md

+# Updating Ruby from source
+
+This guide explains how to update Ruby in case you installed it from source according to the instructions in https://github.com/gitlabhq/gitlabhq/blob/master/doc/install/installation.md#2-ruby .
+
+### 1. Look for Ruby versions
+This guide will only update `/usr/local/bin/ruby`. You can see which Ruby binaries are installed on your system by running:
+
+```bash
+ls -l $(which -a ruby)
+```
+
+### 2. Stop GitLab
+
+```bash
+sudo service gitlab stop
+```
+
+### 3. Install or update dependencies
+Here we are assuming you are using Debian/Ubuntu.
+
+```bash
+sudo apt-get install build-essential zlib1g-dev libyaml-dev libssl-dev libgdbm-dev libreadline-dev libncurses5-dev libffi-dev curl
+```
+
+### 4. Download, compile and install Ruby
+Pick a Ruby version at https://www.ruby-lang.org/en/downloads/ . Here we use Ruby 2.0.0p353, which is patched against [CVE-2013-4164](https://www.ruby-lang.org/en/news/2013/11/22/heap-overflow-in-floating-point-parsing-cve-2013-4164/).
+
+```bash
+cd /tmp
+curl --progress http://cache.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p353.tar.gz | tar xz
+cd ruby-2.0.0-p353
+./configure --disable-install-rdoc
+make
+sudo make install # overwrite the existing Ruby in /usr/local/bin
+```
+
+### 5. Reinstall GitLab gem bundle
+Just to be sure we will reinstall the gems used by GitLab. Note that the `bundle install` command [depends on your choice of database](https://github.com/gitlabhq/gitlabhq/blob/master/doc/install/installation.md#install-gems).
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H rm -rf vendor/bundle  # remove existing Gem bundle
+sudo -u git -H bundle install --deployment --without development test postgres aws # Assuming MySQL
+```
+
+### 6. Start GitLab
+We are now ready to restart GitLab.
+
+```bash
+sudo service gitlab start
+```
+
+### Done