[docs] Streamline documentation.

Previously we had a lot of documentation bundled in the `workflows.md`
file, which wasn't necessarily obvious to new users. The getting the
code documentation now lives in a dedicated file `get_the_code.md`,
which is also directly linked from the toplevel `README.md`, since
that's what developers are looking first.

It also rearranges the rest of the project documentation a bit to move
the most important things to the top, add [TOC]s everywhere and
streamlines the documentation around (3rd party) dependencies, as well
as updating the documentation on testing.

Bug: none
Change-Id: I7b5d9121c7f1137eeff7a4ff159816908ef10705
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/4931037
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Philip Pfaffe <pfaffe@chromium.org>

docs/contributing_changes.md

@@ -0,0 +1,86 @@
+# Contributing changes to Chromium DevTools
+
+See [Get the Code](get_the_code.md) for details on how to checkout the code, and [Chrome DevTools Design Review Guidelines](design_guidelines.md) for
+information regarding our design process.
+
+[TOC]
+
+## Creating a change
+
+Usual [steps](https://chromium.googlesource.com/chromium/src/+/main/docs/contributing.md#creating-a-change) for creating a change work out of the box, when executed in the DevTools frontend repository.
+
+Tips to create meaningful CL descriptions:
+- Provide information on what was changed and why
+- Provide before/after screenshots (if applicable)
+- Provide relevant link to demo or example (if applicable)
+- Provide link to design doc (if applicable)
+
+Example CL, adapted from [Chromium guidelines](https://chromium.googlesource.com/chromium/src/+/main/docs/contributing.md#uploading-a-change-for-review):
+
+```
+Summary of change (one line)
+
+Longer description of change addressing as appropriate:
+what change was made, why the change is made, context if
+it is part of many changes, description of previous behavior
+and newly introduced differences, etc.
+
+Long lines should be wrapped to 72 columns for easier log message
+viewing in terminals.
+
+How to test:
+  1. ..
+  2. ..
+
+Before:  https://page-to-before-screenshot.com/before
+After:  https://page-to-after-screenshot.com/after
+Bug: 123456
+
+```
+## Merges and cherry-picks
+
+_Merge request/approval is handled by Chromium Release Managers. DevTools follows [Chromium's merge criteria](https://chromium.googlesource.com/chromium/src.git/+/refs/heads/main/docs/process/merge_request.md#merge-criteria-phases). In exceptional cases please get in touch with hablich@chromium.org._
+
+Step-by-step guide on how to merge:
+
+1. Request approval to merge by adding the `Merge-Request-XX` label to the relevant crbug. A bot will come by and either ask for more info ([example](https://bugs.chromium.org/p/chromium/issues/detail?id=1123307#c1)) or approve the request.
+1. Backmerges are done to the `chromium/xxxx` (e.g. `chromium/3979`) branch on the DevTools frontend repo.
+   Use <https://chromiumdash.appspot.com/branches> or [Omahaproxy](https://omahaproxy.appspot.com/)
+   to find out what branch a major Chromium version has (column `true_branch`).
+1. Open the to-be-merged commit in Gerrit
+   ([example](https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1928912)).
+1. Click the hamburger menu on the top right and select “Cherry pick”.
+1. Select the branch to merge to e.g. `chromium/3968`.
+1. The cherry-pick CL is created
+   ([example](https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1928913)).
+1. Get it reviewed if necessary.
+1. Once merge request approval is granted (see step 1), click the hamburger menu on the cherry-pick CL and select “Submit”. (Setting the Commit-Queue bit (+2) has no effect because these branches don’t have a commit queue.)
+1. Done.
+
+### Merge conflicts
+
+If the approach above causes conflicts that need resolving, you can use an alternative git workflow which allows you to resolve conflicts locally before uploading. This is very similar to the [chromium git merge steps](https://chromium.googlesource.com/chromium/src.git/+/refs/heads/main/docs/process/merge_request.md#using-git) but with different branch names. These steps will **create the cherry-pick CL via git**.
+
+_It is suggested to use the Gerrit UI approach when possible, it is more straightforward and automated. Only use this approach if your cherry-pick causes conflicts._
+
+For the commands below, replace `xxxx` with the Chromium branch number that you are merging into.
+
+To set up your local environment run:
+
+```
+gclient sync --with_branch_heads
+git fetch
+git checkout -b BRANCH_NAME origin/chromium/xxxx
+git cl upstream origin/chromium/xxxx
+```
+
+You can then cherry-pick your commit from the main branch:
+
+```
+git cherry-pick -x YOUR_COMMIT
+```
+
+You can then resolve any conflicts, run tests, build DevTools, etc, locally to verify everything is working. Then run `git cl upload` to upload the CL and get a review as normal.
+
+**Make sure you remove the Change-ID: line** from the description to avoid issues when uploading the CL.
+