Firebird: add info FDB/FBK use on README

Thank you again Mike for this info!

Change-Id: Iab02f486035de48636d84148a045e86b24307a95
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/151218
Tested-by: Julien Nabet <serval2412@yahoo.fr>
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>

connectivity/README.md

@@ -48,3 +48,19 @@ the environment variable "CONNECTIVITY_TEST_MYSQL_DRIVER".
     podman stop mariadb
     podman rm mariadb
 ```
+
+### Firebird
+
+Firebird has two primary file types:
+
+- Databases - FDB files. These are version-specific, platform-specific, optimized for performance, and thus incompatible between versions. These are what those comments are about. Initially, when FB integration was considered, these files were evaluated for ODBs, but were rejected because of the said incompatibility - even when the version is the same, it will differ on big endian architecture and little endian one. The problem discussed in those comments is when people open stand-alone FDBs that are shipped e.g. with FB installation itself, not when people open ODBs.
+
+- Database backups - FBKs. These are what we use inside ODBs. These are designed to be compatible, independent of architecture; and later versions of FB are always able to open FBKs created in older FB versions.
+
+Our embedded FB is used like this:
+- FBK is extracted from ODB;
+- Embedded FB extracts the compatible FBK into an incompatible FDB (specific to this version of embedded FB DLL);
+- FB works with this temporary FDB;
+- When saving ODB, embedded FB backups the FDB into FBK again, and that is stored inside the ODB.
+
+It, indeed, creates additional performance penalty, but makes the ODB readable by all the future LO versions, no matter what future FB version they embed.