Add build instructions.

BUILDING.md

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+Building AFPBridge
+==================
+
+AFPBridge is designed to be built under GNO 2.0.6, with ORCA/C and ORCA/M
+installed under `/lang/orca` as described in the GNO documentation.
+I am using a custom version of ORCA/C with several patches applied,
+but I believe a stock version of ORCA/C 2.1.x should also work.
+
+You also need to get the `AppleTalk.h` header file, which is included
+under `Libraries/APWCInclude` in an ORCA/C installation.  Copy it either
+to `/lang/orca/Libraries/ORCACDefs` or to the directory with the AFPBridge
+source files.  The original version of that file does not include
+prototypes in its function declarations, which will cause an ORCA/C error
+with the settings in the makefile.  To avoid this, either remove the 
+`-w` flag from `CFLAGS` in `Makefile.mk`, or add the prototypes in
+`AppleTalk.h`.  If adding the prototypes, they should be `RamForbid(void)`,
+`RamPermit(void)`, and `_CALLAT(void*)`.
+
+To build AFPBridge using source files copied directly from the Git repository,
+first run:
+
+    make import
+
+This sets the file types appropriately, converts files to Apple II-style
+line endings, and generates required assembly-language macro files.
+
+Once that is done, you can build the code by running:
+
+    make
+
+This builds the `AFPBridge` init, the `AFPMounter` CDev, and several
+command-line utilities that can be useful for testing and debugging.
+
+You can also run `make install` to install the init and CDev in your
+system folder, or `make clean` to remove the generated files.