Add some info about binary interface to design docs

docs/design.md

@@ -389,3 +389,35 @@ Possible solutions:
     are always printed as int64 values, and a new format
     "@1234567890123456" is added to the parser for specifying them
     exactly.
+
+Binary interface
+----------------
+
+The ASCII interface is too slow for high-bandwidth processing, like
+sinefits, prep, etc.  A binary interface was added so that you can
+extract the raw binary out of the bulkdata storage.  This binary is
+a little-endian format, e.g. in C a uint16_6 stream would be:
+
+    #include <endian.h>
+    #include <stdint.h>
+    struct {
+        int64_t timestamp_le;
+        uint16_t data_le[6];
+    } __attribute__((packed));
+
+Remember to byteswap (with e.g. `letoh` in C)!
+
+This interface is used by the new `nilmdb.client.numpyclient.NumpyClient`
+class, which is a subclass of the normal `nilmcb.client.client.Client`
+and has all of the same functions.  It adds three new functions:
+
+- `stream_extract_numpy` to extract data as a Numpy array
+
+- `stream_insert_numpy` to insert data as a Numpy array
+
+- `stream_insert_numpy_context` is the context manager for
+  incrementally inserting data
+
+It is significantly faster!  It is about 20 times faster to decimate a
+stream with `nilm-decimate` when the filter code is using the new
+binary/numpy interface.