Summary of resolved issues/ changes:
1. 'our' typed twice under **Join us** heading in README.md --> 'our ' is no longer repeated, typo fixed
2. verb form and comma related errors under **Memory Safety** heading in README.md --> grammar fixed
3. missing article and hyphen related errors under **Generics** heading in README.md --> grammar fixed
4. punctuation and preposition related errors under **Why build Carbon?** heading in README.md --> grammar fixed
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
