Define file format (version 0 file format)

Resolves #1.

diff --git a/file_format.md b/file_format.md
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+# File Format
+
+## Format Version 0
+
+File extension is "*.simplearchive"
+
+First 18 bytes of file will be:
+
+    SIMPLE_ARCHIVE_VER
+
+Next 2 bytes is 16-bit unsigned integer "version" in big-endian. In this case,
+will be zero.
+
+Next 4 bytes is 32-bit unsigned integer "file count" in big-endian which will
+indicate the number of files in this archive.
+
+For each file:
+
+1. 2 bytes is 16-bit unsigned integer "filename length" in big-endian.
+2. X bytes of filename (defined by previous value).
+3. 8 bytes 64-bit unsigned integer "location of filename in this archive file".
+4. 8 bytes 64-bit unsigned integer "size of filename in this archive file".
+
+The remaining bytes in the file are the files to be included in the archive file
+concatenated together. Their locations and sizes should match what was listed
+before.