chore: introduce PR template (#579)

Signed-off-by: Jan Kowalleck <[email protected]>

Author: jkowalleck

.github/pull_request_template.md

@@ -0,0 +1,18 @@
+<!-- 
+Thank you for taking the time to develop and contribute a core enhancement or fix for a defect!
+
+We kindly request that you create pull requests only for things that have been discussed in a ticket first; exceptions may be made for spelling or grammar fixe.
+Please have the related ticket/issue ID ready. 
+If there is none, feel free to create a new ticket: https://github.com/CycloneDX/specification/issues/new/choose
+-->
+
+<!-- 
+
+Please provide a brief description of what this pull request intends to do and which ticket it fixes/closes.
+
+Example: 
+> As discussed in ticket #485, this PR adds Streebog to the hash algorithm enum. 
+> 
+> fixes #485 
+
+-->