Public facing pages, including readmes and translation files, may not be used to spam. Spammy behavior includes (but is not limited to) unnecessary affiliate links, tags to competitors plugins, use of over 12 tags total, blackhat SEO, and keyword stuffing.
Links to directly required products, such as themes or other plugins required for the plugin's use, are permitted within moderation. Similarly, related products may be used in tags but not competitors. If a plugin is a WooCommerce extension, it may use the tag 'woocommerce.' However if the plugin is an alternative to Akismet, it may not use that term as a tag. Repetitive use of a tag or specific term is considered to be keyword stuffing, and is not permitted.
Readmes are to be written for people, not bots.
In all cases, affiliate links must be disclosed and must directly link to the affiliate service, not a redirect or cloaked URL.