Separate Readme sections for Source mode, Reflect mode and Flags

[dstivanov]

Description

As a newbie in the tooling I was not able to find out that there are two modes of generation (Source and Reflect) from first glance. I believe that this small change can help the other like me.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests - N/A

Reviewer Notes

• The code flow looks good.
• Tests added.

Release Notes

N/A - no release changes, only markdown changes

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[dstivanov]

@googlebot I signed it

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[google-cla]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[codyoss]

Thanks for the PR, just a couple small nits.