Hi, I'd eventually like to contribute in some way but I encountered a problem.

Brief of question

This is my first time participating in open source work. So I searched for the correct way to submit a PR. Then I found the most appropriate path forward would be to fork, test, and build out of one git branch(e.g. the master branch). But after reading the CONTRIBUTING.md,
I was confused about which git branch should I commit to: master or 4.17.15-post.

The ways I had tried to solve this problem:

1. Read CONTRIBUTING.md
   In the Contributing to Lodash section I found:

Contributions are always welcome. Before contributing please read the code of conduct & search the issue tracker; your issue may have already been discussed or fixed in master. To contribute, fork Lodash, commit your changes, & send a pull request.

It seems like I should checkout a new branch based master, but then I found that in the section Pull request:

For additions or bug fixes you should only need to modify lodash.js. Include updated unit tests in the test directory as part of your pull request.
...
Before running the unit tests you’ll need to install, npm i, development dependencies. Run unit tests from the command-line via npm test, or open test/index.html & test/fp.html in a web browser.

I didn't find lodash.js under the master branch, and npm run test also didn't work because of the lack of files needed for building and testing.
But I found the lodash.js under the 4.17.15-post branch, and npm run test could work .

So, if I want to fix a bug, should I develop based on the 4.17.15-post branch?

Then I found the following paragraph in the Notice section:

Pardon the mess. The master branch is in flux while we work on Lodash v5. This means things like npm scripts, which we encourage using for contributions, may not be working. Thank you for your patience.

Does this mean contributions should not commit to the master branch for now?

2. Search recent pull requests
   To figure it out, I checked some recent pull requests, and found that some were commited based on the master branch (PR#4681), and some were commited based on 4.17.15-post branch (PR#4515), so I was completly confused:

Detail of question

1. If I want to fix a bug, which branch should be used for development?

2. If I only modify a document file like REAMDE.md or CONTRIBUTING.md, which branch should be used to submit?

Hope someone can help.