Create Linux readme file

linux.md

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+# Linux
+
+## Structure
+
+There are 3 projects
+
+* libapple: the core emulator files
+* applen: a frontend based on ncurses
+* qapple: Qt frontend
+
+The libapple interface is a *link time* interface: some functions are not defined and must be provided in order to properly link 
+the application. These functions are listed in [interface.h](source/linux/interface.h).
+
+The main goal is to reuse the AppleWin source files without changes: only where really necessary the AppleWin source files have
+been modified, mostly for
+
+* header files issues
+* const char *
+* STL constructor issues (see Disk_t and HDD)
+* exclude some Windows heavy blocks (source/MouseInterface.cpp)
+
+##  What works
+
+Some key files have been completely reimplemented or discarded:
+
+* AppleWin.cpp
+* Video.cpp
+* Frame.cpp
+* NTSC.cpp
+
+Some features totally ignored:
+
+* NSTC colors
+* ethernet
+* serial port
+* sound
+* debugger
+* speech
+
+The rest is in a usable state.
+
+### applen
+
+Frontend based on ncurses, with a ASCII art graphic mode.
+
+Keyboard shortcuts
+
+* F2: terminate emulator
+* F12: Load Snapshot
+* ALT-LEFT: 2x narrower hi res graphics
+* ALT-RIGHT: 2x wider hi res graphis
+* ALT-UP: 2x vertical
+* ALT-DOWN: 2x vertical
+
+The joystick uses evdev.
+
+### qapple
+
+This is based on Qt, currently tested with 5.10
+
+* keyboard shortcuts are listed in the menu entries
+* graphics: code borrowed from linapple, no NTSC color
+  * lo res in color
+  * hi res in BW
+* joystick: it uses QtGamepad (correct names will only be displayed with 5.11)
+* emulator runs in the main UI thread
+* Qt timers are very coarse: the emulator needs to dynamically adapt the cycles to execute
+* full speed when disk spins execute up to 5 ms real wall clock of emulator code (then returns to Qt)
+
+## Build
+
+The project can be built using cmake.
+
+qapple can be managed from Qt Creator as well and the 2 have coexisted so far, but YMMV.
+The file version.hpp is generated by cmake.