add section on writing Nix expressions

that removes pressure to properly introduce the Nix language while
teaching the concept of declarative environments, and hopefully better
separates concerns.

README.md

@@ -48,7 +48,7 @@ flowchart
 	subgraph book[The Nix Book]
 		direction TB
 		build --> imperative-pkgs & declarative-pkgs
-		declarative-pkgs --> declarative-config & dev & nixpkgs-learn
+		declarative-pkgs --> writing --> declarative-config & dev & nixpkgs-learn
 		subgraph build[run software]
 			direction LR
 			find-pkg[find packages]
@@ -70,7 +70,7 @@ flowchart
 			pin-deps[pin dependencies]
 			gc[garbage collection]
 		end
-		subgraph nixpkgs-learn[Nix package collection]
+		subgraph nixpkgs-learn[package collection]
 			direction LR
 			modify[modify existing package]
 			newpkg[create new package]
@@ -82,6 +82,14 @@ flowchart
 			os[operating system distribution]
 			services[service management]
 		end
+		subgraph writing[configuration language]
+			syntax
+			functions
+			attrsets[attribute sets]
+			debugging
+			library
+			tools[tool support]
+		end
 		subgraph dev[software development]
 			direction LR
 			lang-ecosystems[language ecosystems]

contents/README.md

@@ -88,8 +88,17 @@ This section shows how to have all of that by declaring a Nix expression in a fi
 - compose packages
 - adapt packages to your needs
 
-This chapter should have at least a reference to or a full-blown copy of a good introduction to the Nix langauge as a prerequisite for working through examples.
-Another option would be to develop an approach to the language by motivating language features through the examples themselves, but that would probably be a lot more work.
+### configuration language
+
+- syntax
+- functions
+- attribute sets
+- debugging
+- library
+- tool support
+  - syntax highlighting
+  - linters
+  - formatters
 
 ### maintain package collection