tap: add README to tests directory

format/tap/testdata/README.md

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+### basic_prog1.tap
+
+The `basic_prog1.tap` test file was created directory from the FUSE emulator.
+
+Inside the emulated ZX Spectrum a BASIC program was created:
+
+```
+10 PRINT "fq is the best!"
+20 GOTO 10
+```
+
+and saved to tape:
+
+```
+SAVE "fqTestProg", LINE 10
+```
+
+Then from FUSE select the menu item `Media > Tape > Save As..`.
+
+Any BASIC, machine code, screen image, or other data, can be saved directly
+using the `SAVE` command. Further instructions can be found here:
+https://worldofspectrum.org/ZXBasicManual/zxmanchap20.html