Update the "Working on balenaEngine" doc

Removed the note saying that only cgroups v1 is supported. Also added
some references to the Engine code, telling where certain features are
implemented.

DEVELOPMENT.md

@@ -31,20 +31,46 @@ Project](https://github.com/moby/moby/) repo.
 ### Unique features
 
 This is an incomplete list of features unique to balenaEngine. I hope to make
-this more complete over time, with pointers to the relevant code.
+this more complete over time.
 
 #### Delta updates
 
 With deltas we allow users to pull only the differences between an image they
 already have (the *basis*) and one they want to have (the *target*). Spares
 bandwidth from users and balena alike!
 
+Relevant code:
+
+* The delta algorithms themselves are implemented in balena's [librsync-go
+  library](https://github.com/balena-os/librsync-go). This is the library that supports
+* On the Engine side, delta creation is implemented in the
+  [`ImageService.DeltaCreate()`](https://github.com/balena-os/balena-engine/blob/2cd17c44b267813dfc1153f7d08306024d1fc032/daemon/images/image_delta.go#L29)
+  function (at `daemon/images/image_delta.go`). This code is pretty much
+  self-contained.
+* Applying deltas is a bit more complicated, as our code is "mixed" with Moby's
+  code. The main point of interest is the
+  `LayerDownloadManager.makeDownloadFunc()` function (at
+  `distribution/xfer/download.go`), particularly the code around the [call to
+  `DecorateWithDeltaPatcher()`](https://github.com/balena-os/balena-engine/blob/2cd17c44b267813dfc1153f7d08306024d1fc032/distribution/xfer/download.go#L364C55-L364C55).
+  In a nutshell, what we have here is a pipeline of operations: downloading the
+  layer data, decompressing it, etc. What we do is adding our own step into this
+  pipeline. This step takes the delta itself on the input and produces the
+  target layer on the output.
+
 #### Resilient image pulls
 
 In the event of network issues while pulling an image, balenaEngine will keep
-trying to resume the interrupted download for some time without the need of
-restarting from scratch. This is very useful for devices working with an
-unstable Internet connection.
+trying to resume the interrupted download without the need of restarting from
+scratch. This is very useful for devices working with an unstable Internet
+connection.
+
+Relevant code: Our changes have been to the [`v2LayerDescriptor.Read()`
+function](https://github.com/balena-os/balena-engine/blob/2cd17c44b267813dfc1153f7d08306024d1fc032/distribution/pull_v2.go#L196)
+(`distribution/pull_v2.go`), which basically implements Go's `Reader` interface
+with data coming from an HTTP source. The idea behind our changes is simple:
+instead of returning an `error` when a download error happens, we return `nil`.
+This will cause the caller to keep trying until the network connectivity is
+reestablished.
 
 #### Alternative delta data root
 
@@ -324,7 +350,6 @@ Finally your should bump the version found in [`VERSION`](./VERSION) to the new
 
 ### Random tips
 
-* As I write this (balenaEngine 20.10.18), we support only cgroups v1.
 * This is something we need to look deeper, but I have seen some errors in
   automated tests when using very recent kernel versions. This happens because
   of changes in some kernel interface. AFAIR, this was fixed upstream, but yet