compiler: notes on updating from upstream

compiler/update-notes.md

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+
+occasionally, it may be required to sync with upstream capn proto changes. some quick notes on the process i've used here, assuming the updated capnproto repo is at CAPNP_CPP:
+
+    $ cp ${CAPNP_CPP}/c++/src/capnp/test.capnp .
+    $ cp ${CAPNP_CPP}/c++/src/capnp/schema.capnp .
+    $ cp ${CAPNP_CPP}/c++/src/capnp/c++.capnp .
+
+fix up `schema.capnp` to reference the in-tree copy of `c++.capnp` (ie, `using Cxx = import "c++.capnp";` at the top of the file)
+
+then, regenerate the schema support:
+
+    $ capnp compile -o ./capnpc-c compiler/schema.capnp
+
+now try to regenerate again, based on the previously regenerated schema.
+
+you can always check the capnpc formatted output during debugging:
+
+    $ capnp compile -ocapnpc-capnpc compiler/schema.capnp