Add CONTRIBUTING.md (#406)

* Add CONTRIBUTING.md

* Include CONTRIBUTING.md in README.md

* Fix broken links

* Update CONTRIBUTING.md

* Update CONTRIBUTING.md

* Use "If a PR" instead of "If the PR"

* Mention fold and pretty printing

* Add missing precedence of the operator '|>'

* Add a note about tests

* Rename to Sophia

* Add missing using keyword

* Update the entire list of keywords in sophia syntax doc

* Add a section about creating a new aesophia release

docs/sophia_syntax.md

@@ -10,8 +10,9 @@ and `*/` and can be nested.
 ### Keywords
 
 ```
-contract elif else entrypoint false function if import include let mod namespace
-private payable stateful switch true type record datatype main interface
+contract include let switch type record datatype if elif else function
+stateful payable true false mod public entrypoint private indexed namespace
+interface main using as for hiding
 ```
 
 ### Tokens