Add Adler32 to hash algorithms (#826)

* Add Adler32 to hash algorithms

Adler32 was added to SPDX 2.3 based on issue 624 (spdx/spdx-spec#624).  This algorithm is not present in the SPDX 3.0 spec.

Signed-off-by: Gary O'Neall <gary@sourceauditor.com>

* Fix RFC syntax and link

Co-authored-by: Arthit Suriyawongkul <arthit@gmail.com>
Signed-off-by: Alexios Zavras (zvr) <zvr+git@zvr.gr>

---------

Signed-off-by: Gary O'Neall <gary@sourceauditor.com>
Signed-off-by: Alexios Zavras (zvr) <zvr+git@zvr.gr>
Co-authored-by: Alexios Zavras (zvr) <github@zvr.gr>
Co-authored-by: Arthit Suriyawongkul <arthit@gmail.com>

model/Core/Vocabularies/HashAlgorithm.md

@@ -18,6 +18,7 @@ practically infeasible to invert.
 
 ## Entries
 
+- adler32: Adler-32 checksum is part of the widely used zlib compression library as defined in [RFC 1950](https://www.rfc-editor.org/info/rfc1950) Section 2.3.
 - blake2b256: BLAKE2b algorithm with a digest size of 256, as defined in [RFC 7693](https://www.rfc-editor.org/info/rfc7693) Section 4.
 - blake2b384: BLAKE2b algorithm with a digest size of 384, as defined in [RFC 7693](https://www.rfc-editor.org/info/rfc7693) Section 4.
 - blake2b512: BLAKE2b algorithm with a digest size of 512, as defined in [RFC 7693](https://www.rfc-editor.org/info/rfc7693) Section 4.