doc: add contributing guide

[anton-trunov]

Issue

Closes #130.

Checklist

• I have updated CHANGELOG.md
• I have run the linter, formatter and spellchecker
• I did not do unrelated and/or undiscussed refactorings

[novusnota]

Splendid work! I only have minor nitpicks to add :)

One suggestion though — to add mention of CONTRIBUTING into the README.md somewhere accessible, so that people can Ctrl/Cmd+f it.

P.S.: I'll complete the PR on exit codes to Tact docs today. That'll make our page much richer than the one in TON docs, so we might include link to our docs here too once that happens

[anton-trunov]

One suggestion though — to add mention of CONTRIBUTING into the README.md somewhere accessible, so that people can Ctrl/Cmd+f it.

good point, done