Re-organize contributor's guide (#4659)

* Reorganize contributor's doc

* Address comments from @trivialfis

* Address @sriramch's comment: include ABI compatibility guarantee

* Address @rongou's comment

* Postpone ABI compatibility guarantee for now

doc/contrib/git_guide.rst

@@ -0,0 +1,76 @@
+###################
+Git Workflow Howtos
+###################
+
+**Contents**
+
+.. contents::
+  :backlinks: none
+  :local:
+
+***********************************
+How to resolve conflict with master
+***********************************
+
+- First rebase to most recent master
+
+  .. code-block:: bash
+
+    # The first two steps can be skipped after you do it once.
+    git remote add upstream https://github.com/dmlc/xgboost
+    git fetch upstream
+    git rebase upstream/master
+
+- The git may show some conflicts it cannot merge, say ``conflicted.py``.
+
+  - Manually modify the file to resolve the conflict.
+  - After you resolved the conflict, mark it as resolved by
+
+    .. code-block:: bash
+
+      git add conflicted.py
+
+- Then you can continue rebase by
+
+  .. code-block:: bash
+
+    git rebase --continue
+
+- Finally push to your fork, you may need to force push here.
+
+  .. code-block:: bash
+
+    git push --force
+
+****************************************
+How to combine multiple commits into one
+****************************************
+Sometimes we want to combine multiple commits, especially when later commits are only fixes to previous ones,
+to create a PR with set of meaningful commits. You can do it by following steps.
+
+- Before doing so, configure the default editor of git if you haven't done so before.
+
+  .. code-block:: bash
+
+    git config core.editor the-editor-you-like
+
+- Assume we want to merge last 3 commits, type the following commands
+
+  .. code-block:: bash
+
+    git rebase -i HEAD~3
+
+- It will pop up an text editor. Set the first commit as ``pick``, and change later ones to ``squash``.
+- After you saved the file, it will pop up another text editor to ask you modify the combined commit message.
+- Push the changes to your fork, you need to force push.
+
+  .. code-block:: bash
+
+    git push --force
+
+*************************************
+What is the consequence of force push
+*************************************
+The previous two tips requires force push, this is because we altered the path of the commits.
+It is fine to force push to your own fork, as long as the commits changed are only yours.
+