Update 'README.md'

README.md

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 # pbkdf2
 
 PBKDF2 hashing for Bash
+
+## USAGE
+
+```
+$ bash pbkdf2.sh *hashtype* *salt* *password* *iterations* *{raw/full/rawfull}*
+```
+
+or
+
+```
+$ ./pbkdf2.sh *hashtype* *salt* *password* *iterations* *{raw/full/rawfull}*
+```
+
+### hashtype *string*
+
+Options: md5, sha1, sha224, sha256, sha384, sha512, blake2b, blake2s, sha3_224, sha3_256, sha3_384, sha3_512, shake_128, shake_256
+
+### salt *string*
+
+This must be a predefind string of A-Z, a-z and 0-9.
+
+### password *string*
+
+This must be a predefind string of A-Z, a-z and 0-9.
+
+### iterations *integer*
+
+Range 1 to 2147483647. The higher the integer the more secure but at the cost of time.
+
+### {raw/full/rawfull} *optional*
+
+This is optional. By default you will get a hash string followed by a new line. If you do not want the new line, say to use in a variable, then use 'raw' to get just the hash in hex. However if you use 'full', you will get a string that can be used to input into a database with a new line at the end. Use 'rawfull' to return the string without the new line.
+
+# What is PBKDF2?
+
+> PBKDF2 applies a pseudorandom function, such as hash-based message authentication code (HMAC), to the input password or passphrase along with a salt value and repeats the process many times to produce a derived key, which can then be used as a cryptographic key in subsequent operations. The added computational work makes password cracking much more difficult, and is known as key stretching. 
+*[Source: Wikipedia](https://en.wikipedia.org/wiki/PBKDF2)*
+
+PBKDF2 is used in many applications where a password needs to be stored for future verification. Unlike just using a straight Hashing Algorithm, PBKDF2 uses a "salt" along side the password, and rehashes repeatedly according to the number of "iterations".
+
+# NOTE:
+
+This script requires Python3 which as it's own license. Please visit https://github.com/python/cpython for more information.
+
+# DONATIONS
+
+Please consider making me small donation. Even though my scripts are open source and free to use, I still need to eat. And the occasional bottle of wine also goes down well.
+
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+
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+
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+
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+
+Your donation contributes to further development.
+
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