Write Contribution Guidelines and add Code of Conduct

[Robbepop]

No description provided.

[athei]

Let's not do github discussion and disable them. They are hard to discover and will only be seen by people following the repo. Better make them go to the forum for discussion.

[Robbepop]

I want to try out and evaluate GitHub Discussions for a while since it is important to open up wasmi for use cases outside of Polkadot smart contracts. If it turns out to be a mess we can still disable it.

[paritytech-cicd-pr]

CRITERION BENCHMARKS

BENCHMARK	MASTER	PR	Diff
execute/count_until	2.2564 ms	2.2086 ms	⚪ -4.0909 %
execute/factorial_iterative	1.5314 µs	1.5189 µs	⚪ -0.7342 %
execute/factorial_recursive	2.0855 µs	2.1001 µs	⚪ +0.7536 %
execute/fib_iterative	9.8994 ms	9.8824 ms	⚪ -0.0545 %
execute/fib_recursive	18.2419 ms	18.3272 ms	⚪ +0.3737 %
execute/global_bump	3.5413 ms	3.5411 ms	⚪ -0.0108 %
execute/host_calls	34.1075 µs	33.0897 µs	⚪ -2.5402 %
execute/memory_fill	8.2114 ms	8.2082 ms	⚪ +0.109 %
execute/memory_sum	7.9344 ms	8.0263 ms	⚪ +0.5464 %
execute/memory_vec_add	14.8639 ms	14.8524 ms	⚪ -0.0737 %
execute/recursive_is_even	2.6939 ms	2.6776 ms	⚪ -0.3352 %
execute/recursive_ok	387.063 µs	384.1998 µs	⚪ -0.5539 %
execute/recursive_scan	490.2071 µs	488.6456 µs	⚪ -0.4595 %
execute/recursive_trap	38.7369 µs	37.9022 µs	🟢 -2.5637 %
execute/regex_redux	3.3811 ms	3.3812 ms	⚪ +0.1278 %
execute/rev_complement	3.0443 ms	3.0041 ms	⚪ -1.3586 %
execute/tiny_keccak	2.6367 ms	2.626 ms	⚪ -0.2582 %
execute/trunc_f2i	3.5363 ms	3.5252 ms	⚪ -1.3233 %
instantiate/wasm_kernel	77.3101 µs	77.5676 µs	⚪ +1.1158 %
translate/wasm_kernel	6.1109 ms	6.104 ms	⚪ -0.2164 %

Link to pipeline