Readme update

[OctopusButtons]

Thank you to @RobLoach, @zoltanvb, @hunterk, @zach-morris, i30817, for helpful information, answers, and discussions.

Database readme update.

Goals

• Add clarity on a few key points (below), to address my own questions and questions I've seen confusion about
• A foundation for some would-be-contributors to become active contributors, and for some Issue Reports to become contributions.
• To publish clarification about clrmamepro header description header in particular, in the hopes of addressing sourcing mysteries for future or current dats.

List of Additions / Expansions

• Haven't deleted any information/points, only added, moved, edited/re-phrased.
• Added intro details about .dat and .rdb files
• Cheats
  • Description expanded a bit, partly to contrast with 'game information data' which is the main topic of the readme
• RetroArch's Usage of the Database outline and description.
  • Validation [...]. Previous text about RetroArch verifying against a known good copy has been re-phrased and moved here.
  • Game Naming [...]
  • Thumbnail images based on game names [...]. Explains that thumbnails are only based on game name (with caveats), not on the database, but that the databased assignment of game names allows featured automated retrieval of thumbnails that otherwise wouldn't happen (without user management of filenames).
  • "Explore" Category Search [...]
  • "Information" view per-game [...]
• Key Field / Matching and index
  • Explanation added: CRC versus Serial, build script code as a reference for which key is in use. (I believe there's a lot of confusion about key matching, with many people imagining it's "hash" not checksum and not serial, and not realizing that in-disc serials are scanned and used for matching, etc. Readme additions intend to address.)
• Precedence
  • Explanation added. Not only for general info but as background for investigating discrepancies.
• Game data specified in dats
  • Brief primer and example added, plus an example of collated metadata
• Header Guidelines for dats
  • Focuses only on the matter of clrmamepro header description field and the current or systematic lack of sourcing/documentation for several dats. Intended to address the situation where, I believe, many people see a typical header description "Manufacturer - Console" and mistakenly think that's required for functionality/by-spec. Specification now given about better documentation of origins, sourcing, including both upstream credit and contributor/scraper/builder and the purpose of the dat if applicable (i.e. what niche or subset or games)
• Repository Contents expansion / "Guide to these dats" (so to speak)
  • Non-exhaustive guide per-folder for "insightful" cases that illustrate patterns or unique niches (fan translations, other variants) that people might be looking for.
  • Explains upstream bulk data in /dat versus in /metadat, partly covers questions of whether a dat is ad hoc / contribute-able Versus whether it's bulk-overwritten by imports, in order to assist/encourage contributions.
  • Pre-emptive databases small note
• Sources
  • Expanded intro to remind that the list highlights the bulk coverage source while subsets of the same library may be covered by manual/smaller fragments in other dats. Added note for ">" precedence.
• Maintenance / Technical Usage
  • Moved the existing admin build/test info here.
  • Changed some header sizes
  • Added brief outline/guideline for Add a New Database, adapted from @RobLoach 's discussions discord, so that it's in a central reference location.
• Contributions
  • Explains upstream data and fixing at the source
  • Small-scale corrections
  • Links back to "Adding a New Database" (above)
  • Note/warning about folder structure revision / housekeeping / consolidation. With hypothetical volunteers in mind.
• Databases & RetroArch Thumbnails
  • Explains that thumbnail repository file names are not updated by database updates, and that thumbnails do not derive from databases. Explains how to contribute fixes to game-name-related thumbnail problems etc.

[zoltanvb]

Wow, that's quite complete :)
Few minor questions added.