Add lesson 49

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>

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+In the last lesson we've declared our UNI strings file under the `Sources` section of the application INF file:
+```
+[Sources]
+  ...
+  Strings.uni
+```
+But within the EDKII codebase you could encounter couple of other methods for including UNI files:
+- as the value for the `MODULE_UNI_FILE` key within the INF file,
+- under the `[UserExtensions.TianoCore."ExtraFiles"]` section of the INF file,
+- as the value for the `PACKAGE_UNI_FILE` key within the DEC file,
+- under the `[UserExtensions.TianoCore."ExtraFiles"]` section of the DEC file.
+
+The most important thing that UNI files included with these methods aren't processed by EDKII build tools! It is the files associated with the UEFI Packaging Specification (https://uefi.org/sites/default/files/resources/Dist_Package_Spec_1_1.pdf). They are used only by the `The Intel(R) UEFI Packaging Tool`. If you don't use it to distribute your packages, you can ignore all these files.
+
+As I don't use it myself, I won't go into the details, but here are the examples how these files look like.
+
+# MODULE_UNI_FILE/[UserExtensions.TianoCore."ExtraFiles"] within the INF file:
+
+Let's look at the https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Universal/ResetSystemPei/ResetSystemPei.inf
+```
+[Defines]
+ ...
+ MODULE_UNI_FILE                = ResetSystemPei.uni
+
+...
+
+[UserExtensions.TianoCore."ExtraFiles"]
+  ResetSystemPeiExtra.uni
+```
+Usually UNI file under the `MODULE_UNI_FILE` key has the same name as the INF file.
+
+And such file contains only 2 strings: `STR_MODULE_ABSTRACT` and `STR_MODULE_DESCRIPTION`.
+Example: https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Universal/ResetSystemPei/ResetSystemPei.uni
+```
+#string STR_MODULE_ABSTRACT             #language en-US "Implements Reset2, ResetFilter and ResetHandler PPIs"
+
+#string STR_MODULE_DESCRIPTION          #language en-US "This driver implements Reset2, ResetFilter and ResetHandler PPIs."
+```
+
+UNI file under the `[UserExtensions.TianoCore."ExtraFiles"]` section usually has the same name as the INF file plus `Extra` postfix.
+
+And such file contains only 1 string: `STR_PROPERTIES_MODULE_NAME`.
+Example: https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Universal/ResetSystemPei/ResetSystemPeiExtra.uni
+```
+#string STR_PROPERTIES_MODULE_NAME
+#language en-US
+"Reset System PEIM"
+```
+
+# PACKAGE_UNI_FILE/[UserExtensions.TianoCore."ExtraFiles"] within the DEC file:
+
+For the example you could look at the https://github.com/tianocore/edk2/blob/master/MdeModulePkg/MdeModulePkg.dec:
+```
+[Defines]
+  PACKAGE_UNI_FILE = MdeModulePkg.uni
+
+...
+
+[UserExtensions.TianoCore."ExtraFiles"]
+  MdeModulePkgExtra.uni
+```
+Here you can see that the naming scheme is similar to the one that was used in the INF files:
+- the file declared as the value of the `PACKAGE_UNI_FILE` key is named exactly like the DEC file,
+- the file declared under the `[UserExtensions.TianoCore."ExtraFiles"]` is named the same plus the `Extra` postfix
+
+Content of these files is also similar to the INF files. The only difference that `<MODULE>` is changed to `<PACKAGE>` in all the string token names:
+
+- MdeModulePkg.uni (https://github.com/tianocore/edk2/blob/master/MdeModulePkg/MdeModulePkg.uni)
+```
+#string STR_PACKAGE_ABSTRACT            #language en-US "Provides the modules that conform to UEFI/PI Industry standards"
+
+#string STR_PACKAGE_DESCRIPTION         #language en-US "It also provides the definitions (including PPIs/PROTOCOLs/GUIDs and library classes) and libraries instances, which are used for those modules."
+```
+- MdeModulePkgExtra.uni (https://github.com/tianocore/edk2/blob/master/MdeModulePkg/MdeModulePkgExtra.uni)
+```
+#string STR_PROPERTIES_PACKAGE_NAME
+#language en-US
+"MdeModule package"
+```
+
+Besides of that you can observe some doc strings for the PCDs in the MdeModulePkg.uni:
+```
+#string STR_gEfiMdeModulePkgTokenSpaceGuid_PcdProgressCodeOsLoaderLoad_PROMPT  #language en-US "Progress Code for OS Loader LoadImage start."
+
+#string STR_gEfiMdeModulePkgTokenSpaceGuid_PcdProgressCodeOsLoaderLoad_HELP  #language en-US "Progress Code for OS Loader LoadImage start.<BR><BR>\n"
+                                                                                             "PROGRESS_CODE_OS_LOADER_LOAD   = (EFI_SOFTWARE_DXE_BS_DRIVER | (EFI_OEM_SPECIFIC | 0x00000000)) = 0x03058000<BR>"
+
+#string STR_gEfiMdeModulePkgTokenSpaceGuid_ERR_80000003 #language en-US "Incorrect progress code provided."
+```
+
+# Conclusion
+
+Again, all these UNI files are irrelevant to the build tools. But these files are present all over the EDKII codebase, so you should at least be familiar with these concepts.