Add .jbang/README.md (#13437)

* Add .jbang/README.md

* Apply suggestions from code review

Co-authored-by: Subhramit Basu <subhramit.bb@live.in>

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Co-authored-by: Subhramit Basu <subhramit.bb@live.in>

Author: koppor

.jbang/README.md

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+# JBang scripts for JabRef
+
+This directory contains [JBang](https://www.jbang.dev/) scripts for JabRef.
+
+Three use cases:
+
+- Try out any pull request with minimal installation. See [our blog entry](https://blog.jabref.org/2025/05/31/run-pr/) for details.
+- Run `JabKit` - JabRef's CLI tool.
+- Run `JabSrv` - JabRef's HTTP server.
+
+## Running `JabKit`
+
+If you have JBang installed, just run `jbang jabkit@jabref --help`
+
+If not, download [`gg.cmd`](https://github.com/eirikb/gg/tree/main?tab=readme-ov-file#ggcmd), place it in the current directory (or on your `PATH`) and execute:
+
+    gg.cmd jbang jabkit@jabref --help
+
+## Running `JabSrv`
+
+If you have JBang installed, just run `jbang jabsrv@jabref --help`
+
+With `gg.cmd`:
+
+    gg.cmd jbang jabsrv@jabref --help
+