docs: added docs detailing how dependencies work

Documentation/dependencies.md

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+# appc Dependencies
+
+Some of the most common questions people new to acbuild ask are related to image
+dependencies. How do they work, how are they referenced, what's the difference
+between `acbuild begin <image>` and `acbuild dependency add <image>`.
+
+This document aims to provide a detailed explanation of image dependencies in
+AppC and how they can be manipulated with acbuild.
+
+## acbuild begin
+
+One way to interact with other images is via `acbuild begin`. Specifics on the
+different ways to invoke it are [here][1].
+
+The short version of it is that when you run `acbuild begin` and provide an
+image, the build will start out containing everything from the specified image.
+It'll have the same manifest, and all the files in the other image will be
+present. When `acbuild write` is called, the resulting image will include all of
+this information, so the final image will be the same size or larger than the
+image you began from.
+
+When this method is used, the original image is copied and modified, and the
+original image isn't needed at runtime when you hand the resulting container
+image to a container runtime.
+
+In AppC terminology, this isn't even using dependencies at all.
+
+## acbuild dependency add
+
+AppC provides a way for images to point to other images needed at runtime; these
+other images are called dependencies. These pointers can be added via the
+`acbuild dep add` command. Specifics on using it are [here][2].
+
+When a container runtime goes to run an image, it also needs all of that image's
+dependencies, and all of the dependencies' dependencies, and so on. Adding a
+dependency to an image simply adds a pointer to a different image, and nothing
+else about the dependency (other than potentially things like labels, and an
+expected hash) are stored in the image being built.
+
+These pointers take the form of AppC image names. When you give an image with
+dependencies to a container runtime, the runtime should first check its local
+store for any images with matching names, use those if they exist, and otherwise
+perform AppC discovery to find the images on the internet and fetch them. AppC
+image names and discovery are described in greater detail [here][3].
+
+acbuild doesn't have a persistent local store, so if acbuild needs to find
+dependencies (which happens if you use the `run` command after a `dep add`
+command) it immediately falls back to performing AppC discovery to find the
+image. This means that local dependencies are not currently supported.
+
+[1]: Documentation/subcommands/begin.md
+[2]: Documentation/subcommands/dependency.md
+[3]: https://github.com/appc/spec/blob/master/spec/discovery.md