Add object spec

OBJECTS.md

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+# Object format
+
+Objects are stored for each mod. Each object consists of a custom binary diff-style format(where numbers are little-endian unless otherwise noted):
+
+```
+0x0B7D1FF0: Magic number
+0x00000000: 32-bit version. By convention, unofficial object formats should have the left-most(commonly referred to as the 31st bit) set.
+0x0000000000000007: 64-bit number of entries
+
+0x000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000: Padding
+
+0xCF83E1357EEFB8BDF1542850D66D8007D620E4050B5715DC83F4A921D36CE9CE47D0D13C5D85F2B0FF8318D2877EEC2F63B931BD47417A81A538327AF927DA3E: SHA512 hash of object with hash replaced with this hash(the SHA512 hash of an empty string)
+
+(entries)
+```
+
+Each entry is in the format:
+
+```
+0x0000000000000007: 64-bit size of path in bytes
+0x0000000000000008: Number of diffs
+0x00: Type of change: 0=Removal of file(no diffs), 1=Modification or addition
+(path as a UTF-8 encoded string using UNIX conventions such as a forward slash / as a separator)
+
+(diffs)
+```
+
+Each diff is in the format:
+
+```
+0x00: Type of change: 0=Removal(no body), 1=Insertion, 2=Replacement
+0x00000000000000: Padding
+0x0000000000000000: Start of change (relative to original file)
+0x0000000000000010: Length of change
+
+(body)
+```
+
+Known versions:
+
+|32-bit value|Name|
+|-----|-----|
+|0x00000000|Official venus version 0.1(this doc)|