doc: Added a fcef format documentation

Author: tarithj

docs/fcef_format.md

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+## FCEF Format specification 
+
+### Ver 1 (v0001) (DEPRECATED)
+This document explains the specification of the FCEF file format Ver 1   
+**DEPRECATED: due to the vulnerability in the Ver 1 encryption algorithm it's strongly recommended to use Ver 2**
+
+The FCEF format contain(s)
+1. nonce & Encrypted Data
+
+**These will be called tags hereafter**     
+
+#### Encrypted data
+A FCEF file generated with FileCrypt writes the nonce and encrypted data **after 0x0**.
+This tag contains the actual data that is used to decrypt.
+
+
+### Ver 2 (v0002)
+This document explains the specification of the FCEF file format Ver 2
+
+The FCEF format contain(s)
+1. Version number
+2. Salt
+3. Nonce & Encrypted data
+
+**These will be called tags hereafter**     
+**The tag size(excluding Encrypted data) of FCEF Ver2 is 22 bytes**
+
+#### Version number
+A FCEF file generated with FileCrypt writes the encryption version that it used to encrypt the file **from 0x0 to 0x5**.
+It's used to check whether the FCEF file can be decrypted using the current version of the software.
+Every version after Ver 1 will contain this tag.   
+**Note: the last byte(0x5) of the version tag contains a line brake(`\n`)**
+**Note: the version number ranges from v0001 to v9999**
+
+#### Salt
+A FCEF file generated with FileCrypt writes the salt generated using `rand.Read(salt)` **from 0x6 to 0x15**
+It's used while decrypting to regenerate the argon2 hash from the password.
+
+#### Encrypted data
+A FCEF file generated with FileCrypt writes the nonce and encrypted data **after 0x15**.   
+This tag contains the actual data that is used to decrypt.
+