adds section headers to Makefile

improves readability

Makefile

+# -----------------------------------------------------------------------------
+# settings
+# -----------------------------------------------------------------------------
+
 # do not use spaces here, instead use either
 # - dashes, as in my-awesome-title, or
 # - underscores, as in my_awesome_title
 TITLE = title
 
+# -----------------------------------------------------------------------------
+# pandoc configuration
+# -----------------------------------------------------------------------------
+
 PANDOC = pandoc
 
 PANDOCFLAGS = \
@@ -11,9 +19,17 @@ PANDOCFLAGS = \
 	--filter pandoc-citeproc \
 	--pdf-engine=xelatex \
 
+# -----------------------------------------------------------------------------
+# sources
+# -----------------------------------------------------------------------------
+
 SOURCES_CSV = $(wildcard *.csv)
 SOURCES_MD = $(sort $(wildcard chapter-*.md))
 
+# -----------------------------------------------------------------------------
+# targets
+# -----------------------------------------------------------------------------
+
 default: all
 
 all: $(TITLE).pdf