Clean up docs for GitHub template actions

[benlk]

Changes

This pull request makes the following changes:

• merges docs/README.md and readme-template.md into README.md
• adds a link that creates a repo-setup issue in the template-generated repo
• improves and simplifies instructions now that we're no longer migrating from per-child-theme submodules on one giant multisite umbrella

Why

For #4

Testing/Questions

Features that this PR affects:

• docs
• initialize.sh

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• Are there loopholes in the docs?

Steps to test this PR:

1. Clone this repo someplace, then run initialize.sh and fill out README.md
2. Click the link in README.md to make sure that it fills out a new issue with an appropriate checklist.

[joshdarby]

@benlk Looking at https://github.com/INN/umbrella-boilerplate/tree/4-docs-and-use-improvements, it looks like the new issue link is broken (it doesn't appear as a link in GH).

I think it needs to be formatted/escaped correctly, like so:

./issues/new?title=Setup+tasks&body=Setup%20tasks%20for%20creating%20the%20boilerplate%20repo%3A%0A%0A-%20%5B%20%5D%20run%20%60.%2Finitialize.sh%60%0A-%20%5B%20%5D%20set%20team%20permissions%0A-%20%5B%20%5D%20remove%20setup%20instructions%20from%20%60README.md%60%0A-%20%5B%20%5D%20update%20%60README.md%60%20for%20this%20site%20to%20generate%20the%20first%20issue%20in%20this%20repo.%20%28Only%20works%20if%20you%27re%20clicking%20it%20from%20the%20root%20of%20the%20newly-created%20repo%20on%20GitHub%29

[benlk]

Updated with your changed URL, and I think it work now?