CONTRIBUTING.md: Add branching policy

[bkeryan]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

Update CONTRIBUTING.md to document the branching policy.

Why should this Pull Request be merged?

Avoid confusion about whether to create release branches.

What testing has been done?

N/A

[bkeryan]

@zhindes @maxxboehme Perhaps we should move some of this content to the wiki and have CONTRIBUTING.md focus on how to make code contributions?

Examples:

• https://github.com/ni/measurement-plugin-python/wiki/Release-Process
• https://github.com/ni/measurement-plugin-labview/wiki/Release-Process

[zhindes]

@zhindes @maxxboehme Perhaps we should move some of this content to the wiki and have CONTRIBUTING.md focus on how to make code contributions?

Examples:

* https://github.com/ni/measurement-plugin-python/wiki/Release-Process

* https://github.com/ni/measurement-plugin-labview/wiki/Release-Process

I like that, but it can be a subsequent change if you like.

[github-actions]

Test Results

    34 files  +    24      34 suites  +24   50m 33s ⏱️ + 47m 24s
 2 348 tests + 2 138   1 994 ✅ + 1 784    354 💤 +  354  0 ❌ ±0 
42 272 runs  +40 172  36 020 ✅ +33 920  6 252 💤 +6 252  0 ❌ ±0 

Results for commit c1ebd62. ± Comparison against base commit 53db026.

♻️ This comment has been updated with latest results.