Add lesson 13

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+In previous lesson we've used `EFI_SHELL_PARAMETERS_PROTOCOL` to get command line parameters to our app.
+It is a valid method but there is a simpler way to do it for the shell apps.
+
+To ease things we can use the entry point in this format:
+https://github.com/tianocore/edk2/blob/master/ShellPkg/Include/Library/ShellCEntryLib.h
+```
+INTN
+EFIAPI
+ShellAppMain (
+  IN UINTN Argc,
+  IN CHAR16 **Argv
+  );
+```
+This way `Argc`/`Argv` are passed directly to the app entry point like it is usually happening in C programming.
+
+Let's create a `SimpleShellApp` based on our `HelloWorld` app.
+
+With the necessary modifications the INF file would look like this:
+```
+[Defines]
+  INF_VERSION                    = 1.25
+  BASE_NAME                      = SimpleShellApp
+  FILE_GUID                      = 2afd1202-545e-4f8d-b8fb-bc179e84ddc8
+  MODULE_TYPE                    = UEFI_APPLICATION
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = ShellCEntryLib
+
+[Sources]
+  SimpleShellApp.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+
+[LibraryClasses]
+  UefiApplicationEntryPoint
+  UefiLib
+  ShellCEntryLib
+```
+Main changes:
+- `ENTRY_POINT = ShellCEntryLib` is added to the `[Defines]` section
+- `ShellCEntryLib` is added to the `[LibraryClasses]` section
+
+In the end it works this way. Shell C library is the main UEFI app with the standard entry point:
+```
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+```
+It parses incoming parameters with the `EFI_SHELL_PARAMETERS_PROTOCOL` like we did it and then calls our app entry point passing all of the parsed parameteres with the call:
+```
+INTN
+EFIAPI
+ShellAppMain (
+  IN UINTN Argc,
+  IN CHAR16 **Argv
+  );
+```
+
+
+To find the necessary `ShellCEntryLib` library class search as usual:
+```
+$ grep ShellCEntryLib -r ./ --include=*.inf | grep LIBRARY_CLASS
+./ShellPkg/Library/UefiShellCEntryLib/UefiShellCEntryLib.inf:  LIBRARY_CLASS                  = ShellCEntryLib|UEFI_APPLICATION UEFI_DRIVER
+```
+
+Add this library class to our `UefiLessonsPkg/UefiLessonsPkg.dsc`:
+```
+ShellCEntryLib|ShellPkg/Library/UefiShellCEntryLib/UefiShellCEntryLib.inf
+```
+Don't forget also to add our new app under the `[Components]` section:
+```
+UefiLessonsPkg/SimpleShellApp/SimpleShellApp.inf
+```
+
+
+Now let's look at the `*.c` file.
+
+We can't use our first print method as `SystemTable` now is unavailable, but we can simply use `SystemTable` with the help of `gST` pointer:
+```
+#include <Library/UefiBootServicesTableLib.h>
+#include <Library/UefiLib.h>
+
+INTN EFIAPI ShellAppMain(IN UINTN Argc, IN CHAR16 **Argv)
+{
+  Print(L"Argc=%d\n", Argc);
+//  SystemTable->ConOut->OutputString(SystemTable->ConOut, L"Hello World!\n");
+  gST->ConOut->OutputString(gST->ConOut, L"Hello again!\n");
+  Print(L"Bye!\n");
+  return 0;
+}
+```
+
+Let's add some parameters handling code:
+```
+for (UINTN i=Argc; i>0; i--) {
+  Print(L"Arg[%d]=%s\n", Argc-i, Argv[Argc-i]);
+}
+```
+
+If we test our app under OVMF we would get:
+```
+FS0:\> SimpleShellApp.efi kkk ggg
+Hello again!
+Bye!
+Arg[0]=FS0:\SimpleShellApp.efi
+Arg[1]=kkk
+Arg[2]=ggg
+FS0:\> SimpleShellApp.efi
+Hello again!
+Bye!
+Arg[0]=FS0:\SimpleShellApp.efi
+```