Replace master with v3-dev after the branch switch. (#28287)

CONTRIBUTING.md

@@ -70,7 +70,7 @@ Guidelines for bug reports:
    reported.
 
 2. **Check if the issue has been fixed** — try to reproduce it using the
-   latest `master` or development branch in the repository.
+   latest `v3-dev` or development branch in the repository.
 
 3. **Isolate the problem** — ideally create a [reduced test
    case](https://css-tricks.com/reduced-test-cases/) and a live example.
@@ -157,13 +157,13 @@ project (indentation, accurate comments, etc.) and any other requirements
 
 **Do not edit `bootstrap.css`, `bootstrap-theme.css`, or `bootstrap.js`
 directly!** Those files are automatically generated. You should edit the
-source files in [`/bootstrap/less/`](https://github.com/twbs/bootstrap/tree/master/less),
+source files in [`/bootstrap/less/`](https://github.com/twbs/bootstrap/tree/v3-dev/less),
 [`/bootstrap/scss/`](https://github.com/twbs/bootstrap/tree/v4-dev/scss) (for Bootstrap v4),
-and/or [`/bootstrap/js/`](https://github.com/twbs/bootstrap/tree/master/js) instead.
+and/or [`/bootstrap/js/`](https://github.com/twbs/bootstrap/tree/v3-dev/js) instead.
 
 Similarly, when contributing to Bootstrap's documentation, you should edit the
 documentation source files in
-[the `/bootstrap/docs/` directory of the `master` branch](https://github.com/twbs/bootstrap/tree/master/docs).
+[the `/bootstrap/docs/` directory of the `v3-dev` branch](https://github.com/twbs/bootstrap/tree/v3-dev/docs).
 **Do not edit the `gh-pages` branch.** That branch is generated from the
 documentation source files and is managed separately by the Bootstrap Core Team.
 
@@ -185,8 +185,8 @@ included in the project:
 2. If you cloned a while ago, get the latest changes from upstream:
 
    ```bash
-   git checkout master
-   git pull upstream master
+   git checkout v3-dev
+   git pull upstream v3-dev
    ```
 
 3. Create a new topic branch (off the main project development branch) to
@@ -205,7 +205,7 @@ included in the project:
 5. Locally merge (or rebase) the upstream development branch into your topic branch:
 
    ```bash
-   git pull [--rebase] upstream master
+   git pull [--rebase] upstream v3-dev
    ```
 
 6. Push your topic branch up to your fork:
@@ -215,7 +215,7 @@ included in the project:
    ```
 
 7. [Open a Pull Request](https://help.github.com/articles/using-pull-requests/)
-    with a clear title and description against the `master` branch.
+    with a clear title and description against the `v3-dev` branch.
 
 **IMPORTANT**: By submitting a patch, you agree to allow the project owners to
 license your work under the terms of the [MIT License](LICENSE) (if it