Add section on using rbenv, separate from main installation instructions.

diff --git a/INSTALL.md b/INSTALL.md
index 597940f0b14aa76bd84274ec85326ab2438b6e78..ceb4eccb0acc27802699f6699d69dc2168a218be 100644 (file)
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -21,7 +21,7 @@ of packages required before you can get the various gems installed.
 * Ruby 2.5+
 * PostgreSQL 9.1+
 * ImageMagick
-* Bundler
+* Bundler (see note below about [developer Ruby setup](#rbenv))
 * Javascript Runtime
 
 These can be installed on Ubuntu 18.04 or later with:
@@ -97,9 +97,9 @@ Installing other dependencies:
 * Install jpegoptim: `brew install jpegoptim`
 * Install gifsicle: `brew install gifsicle`
 * Install svgo: `brew install svgo`
-* Install Bundler: `gem install bundler` (you might need to `sudo gem install bundler` if you get an error about permissions)
+* Install Bundler: `gem install bundler` (you might need to `sudo gem install bundler` if you get an error about permissions - or see note below about [developer Ruby setup](#rbenv))
 
-You will need to tell `bundler` that `libxml2` is installed in a Homebrew location. If it uses the system-installed one then you will get errors installing the `libxml-ruby` gem later on.
+You will need to tell `bundler` that `libxml2` is installed in a Homebrew location. If it uses the system-installed one then you will get errors installing the `libxml-ruby` gem later on<a name="macosx-bundle-config"></a>.
 
 ```
 bundle config build.libxml-ruby --with-xml2-config=/usr/local/opt/libxml2/bin/xml2-config
@@ -257,3 +257,27 @@ Note that the OSM map tiles you see aren't created from your local database - th
 # Configuration
 
 After installing this software, you may need to carry out some [configuration steps](CONFIGURE.md), depending on your tasks.
+
+# Ruby development install and versions<a name="rbenv"></a>
+
+For simplicity, this document explains how to install all the website dependencies as "system" dependencies. While this is simpler, and usually faster, you might want more control over the process or the ability to install multiple different versions of software alongside eachother. For many developers, [`rbenv`](https://github.com/rbenv/rbenv) is the easiest way to manage multiple different Ruby versions on the same computer - with the added advantage that the installs are all in your home directory, so you don't need administrator permissions.
+
+If you choose to install Ruby and Bundler via `rbenv`, then you do not need to install the system libraries for Ruby:
+
+* For Ubuntu, you do not need to install the following packages: `ruby2.5 libruby2.5 ruby2.5-dev bundler`,
+* For Fedora, you do not need to install the following packages: `ruby ruby-devel rubygem-rdoc rubygem-bundler rubygems`
+* For MacOSX, you do not need to `brew install ruby` - but make sure you've installed a version of Ruby using `rbenv` before running `gem install bundler`!
+
+After installing a version of Ruby with `rbenv` (the latest stable version is a good place to start), you will need to make that the default. From inside the `openstreetmap-website` directory, run:
+
+```
+rbenv local $VERSION
+```
+
+Where `$VERSION` is the version you installed. Then install bundler:
+
+```
+gem install bundler
+```
+
+You should now be able to proceed with the rest of the installation. If you're on MacOSX, make sure you set up the [config override for the libxml2 location])(#macosx-bundle-config) _after_ installing bundler.