docs: Update README

Give light hints to how the section arrays work.

README.md

@@ -18,6 +18,28 @@ Is called like the following:
 ```
 
 This is the default invocation, though sidebar and main body sections can be exchanged at will.
+The following sections currently exist:
+
+- education
+- experience, subdivided in "experience", "experience_by_client" and "experience_by_type"
+- languages
+- skills
+- summary
+- volunteering
+
+Sections in the main body or sidebar can be reordered at will.
+
+## Advanced experience settings
+
+The experience section has 3 forms:
+
+Purely chronological ("experience"), which is the default;
+separated by client worked for ("experience_by_client");
+separated by the type of work undertaken, then further separated by client worked for ("experience_by_client").
+
+These options are intended especially for self-employed / entrepreneurial CVs.
+They let you subdivide your work experience whichever way works best,
+and additionally divide work undertaken for your own employ or salaried positions for example.
 
 TODO:
 
@@ -42,4 +64,4 @@ TODO:
 - [x] Can be switched between Ger/En
 - [x] Try sidebar version
   - [ ] Generalize sidebar version through abstraction/extractions
-- [ ] unify items: experience/education/(thesis?)
+- [x] unify items: experience/education/(thesis?)