Include RE2 in the upgrade docs

doc/install/installation.md

@@ -66,6 +66,9 @@ Install the required packages (needed to compile Ruby and native extensions to R
 
     sudo apt-get install -y build-essential zlib1g-dev libyaml-dev libssl-dev libgdbm-dev libre2-dev libreadline-dev libncurses5-dev libffi-dev curl openssh-server checkinstall libxml2-dev libxslt-dev libcurl4-openssl-dev libicu-dev logrotate python-docutils pkg-config cmake
 
+Ubuntu 14.04 (Trusty Tahr) doesn't have the `libre2-dev` package available, but
+you can [install re2 manually](https://github.com/google/re2/wiki/Install).
+
 If you want to use Kerberos for user authentication, then install libkrb5-dev:
 
     sudo apt-get install libkrb5-dev

doc/update/8.17-to-9.0.md

@@ -264,6 +264,16 @@ sudo systemctl daemon-reload
 
 ### 9. Install libs, migrations, etc.
 
+GitLab 9.0.11 [introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/24570)
+a dependency on on the `re2` regular expression library. To install this dependency:
+
+```bash
+sudo apt-get install libre2-dev
+```
+
+Ubuntu 14.04 (Trusty Tahr) doesn't have the `libre2-dev` package available, but
+you can [install re2 manually](https://github.com/google/re2/wiki/Install).
+
 ```bash
 cd /home/git/gitlab
 

doc/update/9.0-to-9.1.md

@@ -264,6 +264,16 @@ sudo systemctl daemon-reload
 
 ### 9. Install libs, migrations, etc.
 
+GitLab 9.1.8 [introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/24570)
+a dependency on on the `re2` regular expression library. To install this dependency:
+
+```bash
+sudo apt-get install libre2-dev
+```
+
+Ubuntu 14.04 (Trusty Tahr) doesn't have the `libre2-dev` package available, but
+you can [install re2 manually](https://github.com/google/re2/wiki/Install).
+
 ```bash
 cd /home/git/gitlab
 

doc/update/9.1-to-9.2.md

@@ -222,6 +222,16 @@ sudo systemctl daemon-reload
 
 ### 10. Install libs, migrations, etc.
 
+GitLab 9.2.8 [introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/24570)
+a dependency on on the `re2` regular expression library. To install this dependency:
+
+```bash
+sudo apt-get install libre2-dev
+```
+
+Ubuntu 14.04 (Trusty Tahr) doesn't have the `libre2-dev` package available, but
+you can [install re2 manually](https://github.com/google/re2/wiki/Install).
+
 ```bash
 cd /home/git/gitlab
 

doc/update/9.2-to-9.3.md

@@ -258,6 +258,16 @@ sudo systemctl daemon-reload
 
 ### 12. Install libs, migrations, etc.
 
+GitLab 9.3.8 [introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/24570)
+a dependency on on the `re2` regular expression library. To install this dependency:
+
+```bash
+sudo apt-get install libre2-dev
+```
+
+Ubuntu 14.04 (Trusty Tahr) doesn't have the `libre2-dev` package available, but
+you can [install re2 manually](https://github.com/google/re2/wiki/Install).
+
 ```bash
 cd /home/git/gitlab
 

doc/update/9.3-to-9.4.md

@@ -271,6 +271,16 @@ sudo systemctl daemon-reload
 
 ### 12. Install libs, migrations, etc.
 
+GitLab 9.4 [introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/24570)
+a dependency on on the `re2` regular expression library. To install this dependency:
+
+```bash
+sudo apt-get install libre2-dev
+```
+
+Ubuntu 14.04 (Trusty Tahr) doesn't have the `libre2-dev` package available, but
+you can [install re2 manually](https://github.com/google/re2/wiki/Install).
+
 ```bash
 cd /home/git/gitlab