docs about format of lib/bs/.bsbuild

Author: bobzhang

jscomp/bsb/data_format.md

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+# format about lib/bs/.bsbuild
+
+This file (in binary) contains all file info needed in build.
+
+It is encoded in bsb_db_encode.ml and bsb_db_decode.ml, the format is optimized for performance in decoding where it is used most.
+
+
+- The first 16 chars is digest of the following content
+
+The rest is encoding for each group (source code and test), in most cases, you have one group or two groups (one for lib one for test).
+
+Begining with a new line, the number of total groups (encoded in text format) are encoded.
+
+
+For each group, it starts with a newline and the
+number of modules (text format).
+
+The following are list of modules in sorted order (Ext_string.compare) separated by newline.
+
+The following are list of directories separated by tab character.
+
+The next is a fixed length for module description, its encoding is hard coded in Bsb_db_encode.make_encoding