DOC: Documentation for benchmarking.

doc/devel/Benchmarking.md

+# Manual Java-Side Benchmarking #
+
+Benchmarking can be done using the `TransactionBenchmark` class (in package
+`caosdb.server.database.misc`).
+
+- Single timings can be added to instances of that class via the
+  `addBenchmark(object, time)` method.  Multiple benchmarks for the same object
+  (typically just strings) can be averaged.
+- Benchmarks can be serialized into XML, `Container` and `Query` objects already
+  use this with their included benchmarks to output benchmarking results.
+- To work with the benchmarks of often used objects, use these methods:
+  - `Container.getTransactionBenchmark().addBenchmark()`
+  - `Query.addBenchmark()`