Add some documentation about where the line between the new variants …

…of buildpack-deps is

buildpack-deps/content.md

@@ -12,3 +12,28 @@ knowing beforehand that `ssl.h` is required to build a dependent module.
 # How to use this image
 
 This stack is designed to be the foundation of a language-stack image.
+
+## What's included?
+
+The main tags of this image are the full batteries-included approach.  With
+them, a majority of arbitrary `gem install` / `npm install` / `pip install`
+should be successfull without additional header/development packages.
+
+For some language stacks, that doesn't make sense, particularly if linking to
+arbitrary external C libraries is much less common (as in Go and Java, for
+example), which is where these other smaller variants can come in handy.
+
+### `curl`
+
+This variant includes just the `curl`, `wget`, and `ca-certificates` packages.
+This is perfect for cases like the Java JRE, where downloading JARs is very
+common and necessary, but checking out code isn't.
+
+### `scm`
+
+This variant is based on `curl`, but also adds various source control management
+tools.  As of this writing, the current list of included tools is `bzr`, `git`,
+`hg`, and `svn`.  Intentionally missing is `cvs` due to the dwindling relevance
+it has (sorry CVS).  This image is perfect for cases like the Java JDK, where
+downloading JARs is very common (hence the `curl` base still), but checking out
+code also becomes more common as well (compared to the JRE).