Can I add a .json to a github library so it can be included via the library manager?

I have a driver for a sensor (an ms5611 temp/pressure sensor) that I want to make available via github (GitHub - foodini/arduino_ms5611_spi: An arduino library for driving the ms5611 pressure and temper). I've tried adding the folder to my libraries directory, and the arduino application (Windows 10) isn't finding it. I'd rather just provide a .json that can be included in the Edit->Preferences dialog so it can be installed through the Library Manager like everything else.

How can I make it easy for people to install my library from github? (And, why doesn't the library manager just let you add a github repo url?)

The reason is because you have used the wrong file name for the library.properties metadata file. The filename must be exactly. library.properties, but yours is named library.properties.txt:

Your file browser software may be configured to hide known file extensions, which would cause it to appear to be named "library.properties".

You should also be sure to read the specification for this file format. I spotted a problem with yours right away:
https://arduino.github.io/arduino-cli/latest/library-specification/#libraryproperties-file-format

name - the name of the library. Library names must contain only basic letters (A-Z or a-z) and numbers (0-9), spaces ( ), underscores (_), dots (.) and dashes (-). They must start with a letter or number. They must contain at least one letter.

As the name hints, the "Additional Boards Manager URLs" field of the Arduino IDE's preferences is for the Boards Manager feature that installs Arduino boards platforms. It can not be used for Library Manager, which installs Arduino libraries. Note that Arduino libraries and Arduino boards platforms are two different things.

  • Use the correct metadata file name..
  • Make the metadata specification-compliant.
  • Provide detailed installation instructions in the library's readme.

The Arduino Library Manager is for installing and updating libraries that are listed in the Arduino Library Manager index. It simply isn't meant for that purpose. Arduino CLI does have such a capability:

arduino-cli lib install --git-url https://github.com/foodini/arduino_ms5611_spi

(currently not working for your specific library due to a bug)

But it's easy enough to just use GitHub's Code > Download ZIP button to download a .zip file of the repository and then use the Arduino IDE's Sketch > Include Library > Add .ZIP Library to install it (also supported via arduino-cli lib install --zip-path).

But if you want to make your library easy to install and update, just submit it for inclusion in the Arduino Library Manager index. You can find the instruction for that here:
https://github.com/arduino/library-registry#readme