templates: Update message for new contributors to align with guide.

Also clearly set expectations for following guidelines.

Author: alya

config/templates/contributorAddition.md

@@ -1,9 +1,11 @@
-Welcome to Zulip, @{commenter}! To get started, please take the following three steps **in order**:
+Welcome to Zulip, @{commenter}! By picking up a Zulip issue, you're promising to do your best to follow [Zulip's contributing guidelines](https://zulip.readthedocs.io/en/latest/contributing/index.html). Neglecting to do so wastes maintainer time, and may result in being banned from contributing to Zulip.
 
-1. Complete all the steps in the [Your first codebase contribution](https://zulip.readthedocs.io/en/latest/overview/contributing.html#your-first-codebase-contribution) section of the [Zulip contributor guide](https://zulip.readthedocs.io/en/latest/contributing/index.html) up to "Claiming an issue".
+To get started:
+
+1. Work through all the "Prior to picking up your first issue" sections of the [Zulip contributor guide](https://zulip.readthedocs.io/en/latest/contributing/index.html).
 2. Post a comment here describing your plan for how to approach this issue. Before you claim an issue, you should be confident that you will be able to tackle it effectively.
 3. Accept the invite to collaborate on this repository at https://github.com/{repoOwner}/{repoName}/invitations. To avoid filling your inbox, we recommend [unwatching this repository](https://help.github.com/articles/unwatching-repositories/).
 
-The issue will be assigned to you when all three steps have been completed.
+The issue will be assigned to you when all three steps have been completed. If you get stuck, you can always `@zulipbot abandon` this issue, and pick a different one.
 
 If someone else claims this issue while you're working through the checklist, no worries! You can contribute by providing feedback on their pull request. If you’ve made good progress in understanding part of the codebase, you can also find another “help wanted” issue in the same area to work on.