Skeleton guide for development in our Make system

docs/dev.md

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+==== FIXME ===
+Finish this doc
+
+== Adding a new package
+
+* Create most of the files in the 'any' architecture for cross compiling
+* Copy the directory structure from an existing package
+    * APKG.yml goes into the $(ONL)/packages/base/any/foo/ directory
+    * PKG.yml goes into the $(ONL)/packages/base/$ARCH/foo directory
+        * Create for each $ARCH you intend to support
+* Put code in $(ONL)/packages/base/any/foo/src
+* Put package specific Makefiles in $(ONL)/packages/base/any/builds
+
+* Run `make rebuild` in $(ONL)/packages/base/$ARCH/foo to rebuild the package cache
+    * particularly if you see an error like:
+      """ERROR:onlpm:'Package all does not exist.'"""