For an introduction to GitHub wikis, see here.
Users signed in with GitHub can edit and add pages using a browser or locally. To view the wiki locally, scroll to the bottom of the sidebar, and get the link to clone the wiki, (posting here for convenience: https://github.com/ethereum/wiki.wiki.git), then press CTRL+ALT+T to open a terminal, run git clone https://github.com/ethereum/wiki.wiki.git and then view the files from a file explorer or using cd wiki.wiki; ls; // Enter a command to open a file from a terminal, e.g. gedit Home.md
Do not change the title of a page, as the link for it will change. In particular, if you change the title for a page, any link to it will just direct to a blank page. (In the case of this page, also the Wiki tab in the header of this repo, as well as any link to the Home page, will just direct to a list of pages.) If you really want to change the title, then create a new page with the new title, move the contents of the old page to the new page, and update the old page with a redirect link to the new page. If you want to translate a page, create a new page and translate the original there. Consider previewing your changes before saving them, and if you detect any errors, fix them. If you happen to get directed to a page that doesn't exist with a prompt to create a new page, do that, without changing the title. Then check the history of the newly created page. It may be that there is a history of changes to the page that you just created, with the second most recent change (second to you creating the page) being that someone renamed the page. If so, please fix the page (restore it to the revision before the title was changed or redirect to the new page) and tag the person who renamed the page in this issue here or on Gitter.
Wikipedia has five pillars which provide a good standard for contributing that we can adapt for our needs. If you have experience with editing on Wikipedia, then that will help with knowing how to edit this wiki, although the contribution rules are less strict. Referencing facts is a key writing and proofreading task, as well as checking that information is up-to-date (and updating it if otherwise), correcting grammar, typos, and spelling; and making the wiki comprehensive and easy to understand. Other rules, such as a neutral point of view and no original research are desirable, but may be hard to maintain. Notability is less relevant, this wiki is about all things Ethereum.
The title of the page should start with [Language]: title, e.g. "[German] White Paper" or "[中文] 以太坊Wiki目录". Then add a Table of Contents (ToC) page linking for the language, linking to all pages of that language, and add a link to it in the footer.
License and contributor license agreement
Please permit your contributions to be under the CC0 license 1, which makes your contributions have no rights reserved, putting them in the public domain. This will help to allow for the dissemination of information about Ethereum, the Ethereum ecosystem and Web 3 to the public, in a completely permissive manner. To state that you accept your contributions to be under a CC0 license, please add yourself to the list of external contributors here. Otherwise, without adding yourself to the list, your contributions will be all rights reserved. Some previous contributions may have all rights reserved (by contributors that have not agreed to a CC0 license at pull request 528), since no copyright permission was stated explicitly. Until all previous contributors agree to a CC0 license, and to provide clarity of licensing, you may also wish to add a HTML comment to the top of pages or sections that you contribute to, like so: <!--*Name Surname*, Github username, **email@domain** and/or other contact methods-->.
For more information, see https://github.com/ethereum/wiki/wiki.
If you had an issue that has been closed, and feel that it could be reopened after following the above steps, feel free to request to reopen it. About 180 issues and 30 PRs were closed on May 29 2018, due to deleting the codebase and referring people to this wiki, in order to reduce the burden on maintenance by following the above steps. This fits well with Ethereum's ethos of decentralization, which includes minimising bureaucracies, and gives contributors more time to build Ethereum and satisfy the long-term interests of current and future users. If you had an issue or PR that was closed without explanation, I apologize, but it is time-consuming to reply to every one.