Add a makefile (#28)

I found myself using ZSH history search to remember the specific `docc`
commands needed to start a preview server and to build a documentation
archive (`.doccarchive` bundle). Another possible approach would be to
create a build script, although a build script wouldn't be able to skip
rebuilding when source files haven't changed.

Author: amartini51

Makefile

@@ -0,0 +1,20 @@
+# This makefile uses only POSIX 'make' features, except for the
+# following widely implemented extensions -- the .PHONY target, which
+# marks targets that don't generate a file whose name is the same as the
+# target, and prerequisites that depend on glob expansion.
+
+.POSIX:
+.SUFFIXES:
+
+.DEFAULT: preview
+.IGNORE: preview
+.PHONY: preview
+preview:
+	swift package --disable-sandbox preview-documentation
+
+.PHONY: archive
+archive: .build/plugins/Swift-DocC/outputs/TSPL.doccarchive
+
+.build/plugins/Swift-DocC/outputs/TSPL.doccarchive: Sources/TSPL/TSPL.docc/*/*.md
+.build/plugins/Swift-DocC/outputs/TSPL.doccarchive: Sources/TSPL/TSPL.docc/Assets/*.png
+	swift package generate-documentation