docs: add faq

Author: woile

docs/faq.md

@@ -0,0 +1,28 @@
+## Support for PEP621
+
+PEP621 establishes a `[project]` definition inside `pyproject.toml`
+
+```toml
+[project]
+name = "spam"
+version = "2020.0.0"
+```
+
+Commitizen **won't** use the `project.version` as a source of truth because it's a
+tool aimed for any kind of project.
+
+If we were to use it, it would increase the complexity of the tool. Also why
+wouldn't we support other project files like `cargo.toml` or `package.json`?
+
+Instead of supporting all the different project files, you can use `version_files`
+inside `[tool.commitizen]`, and it will cheaply keep any of these project files in sync
+
+```toml
+[tool.commitizen]
+version = "2.5.1"
+version_files = [
+    "pyproject.toml:^version",
+    "cargo.toml:^version",
+    "package.json:\"version\":"
+]
+```

mkdocs.yml

@@ -21,6 +21,7 @@ nav:
         - Writing commits: 'tutorials/writing_commits.md'
         - GitLab CI: 'tutorials/gitlab_ci.md'
         - Github Actions: 'tutorials/github_actions.md'
+    - FAQ: 'faq.md'
     - Exit Codes: 'exit_codes.md'
     - Third-Party Commitizen Templates: 'third-party-commitizen.md'
     - Contributing: 'contributing.md'