Add MRI3.2 to supported versions documentation

[niku]

There are some PRs/Codes that look like the Spring has been already supported MRI3.2. For example:

• https://github.com/rails/spring/blob/3fd5c16a4693fbcc7f0840939e225a40c5f250aa/.github/workflows/ci.yml#L9
• https://github.com/rails/spring/pull/691

However, documents does not have text that shows support for MRI3.2.