Documentation: Add in a CITATION.cff file

Initial addition of a CITATION file, no version information was included
because keeping it up to date with the version in the repo automatically
would be tricky and not doing so would lead it to becoming out of sync.

Fixes #1013.

CITATION.cff

@@ -0,0 +1,37 @@
+# This CITATION.cff file was generated with cffinit.
+# Visit https://bit.ly/cffinit to generate yours today!
+
+cff-version: 1.2.0
+title: Volatility 3
+message: >-
+  If you reference this software, please feel free to cite
+  it using the information below.
+type: software
+authors:
+  - name: Volatility Foundation
+    country: US
+    website: 'https://www.volatilityfoundation.org/'
+identifiers:
+  - type: url
+    value: 'https://github.com/volatilityfoundation/volatility3'
+    description: Volatility 3 source code respository
+repository-code: 'https://github.com/volatilityfoundation/volatility3'
+url: 'https://github.com/volatilityfoundation/volatility3'
+abstract: >-
+  Volatility is the world's most widely used framework for
+  extracting digital artifacts from volatile memory (RAM)
+  samples. The extraction techniques are performed
+  completely independent of the system being investigated
+  but offer visibility into the runtime state of the system.
+  The framework is intended to introduce people to the
+  techniques and complexities associated with extracting
+  digital artifacts from volatile memory samples and provide
+  a platform for further work into this exciting area of
+  research.
+keywords:
+  - malware
+  - forensics
+  - memory
+  - python
+  - ram
+  - volatility