[skip changelog] Fully document library layouts in specification

Arduino library authors may chose from multiple structures, each of which have different implications for the compilation
of the library source files. The specification explains this in a somewhat simplistic manner as "1.5 format" and
"1.0 format". That is sufficient for most, but does not provide a complete picture. Feedback indicated that more advanced
users were left wanting complete documentation, which is provided here.

The subject is not well suited for description via prose, so a chart and table are employed instead.

The chart is defined in the DOT graph description language.

Author: per1234

docs/img/library-layout.dot

@@ -0,0 +1,39 @@
+// See: https://www.graphviz.org/documentation/
+digraph {
+  // This generates library-layout.png using the following command:
+  // dot -Tsvg -olibrary-layout.svg library-layout.dot
+  graph [
+    splines=line
+  ]
+  node [
+    shape=box
+  ]
+
+  subgraph {
+    // This subgraph provides the labels on left margin
+    node [
+      margin="0.0,0.0"
+      shape=none
+      width=0.0
+    ]
+    "Metadata:" -> "Format:" -> "Structure:" -> "Layout:" [style=invis]
+  }
+
+  subgraph cluster1 {
+    peripheries=0
+    "library.properties" [height=1.0 margin="0.0,0.0" shape=diamond width=0.0]
+    "library.properties" -> "1.5" [label="yes"]
+    "library.properties" -> "1.0" [label="no"]
+    subgraph cluster2 {
+      peripheries=0
+      rank=same
+      "1.5"
+      "1.0"
+    }
+    "1.5" -> "src/"
+    "src/" [shape=diamond]
+    "src/" -> "recursive" [label=" yes"]
+    "src/" -> "flat" [label="no"]
+    "1.0" -> "flat"
+  }
+}

docs/img/library-layout.svg

@@ -163,6 +163,15 @@ only needs to run on Arduino IDE 1.5.x+, we recommend placing all source code in
 recursive compilation of nested source folders, its code must be in the src/ folder (since Arduino IDE 1.0.x doesn’t
 support recursive compilation, backwards compatibility wouldn’t be possible anyway).
 
+##### Library layout
+
+![Library layout](img/library-layout.svg)
+
+| Layout        | root compiled | `src` compiled | `utility` compiled |
+| ------------- | ------------- | -------------- | ------------------ |
+| **recursive** | no            | recursively    | no                 |
+| **flat**      | yes           | no             | yes                |
+
 #### Precompiled binaries
 
 The `precompiled` field of [library.properties](#libraryproperties-file-format) enables support for the use of