new: add some basic documentation about version dectction

CMakeLists.txt

@@ -124,7 +124,7 @@ elseif(USE_VERSION_FROM_GIT_TAG)
 endif()
 
 if (NOT GIT_REVISION)
-	set(GIT_REVISION ${FREEERDP_VERSION})
+	set(GIT_REVISION ${FREERDP_VERSION})
 endif()
 
 message(STATUS "Git Revision ${GIT_REVISION}")

docs/version_detection.md

@@ -0,0 +1,35 @@
+As FreeRDPs is build on different OS with different build tools and methods the
+"version detection" has grown historically.
+This document quickly describes how it's currently used.
+
+When doing a `xfreerdp /version` for example the following is shown
+
+`This is FreeRDP version 3.0.0-dev (c99c4cecddee4e5b914b122bc1531d47a668bb8e)`
+
+The first part ist the Version as defined in `RAW_VERSION_STRING` and the second part, in braces,
+the `GIT_REVISON` of this version.
+
+`RAW_VERSION_STRING` is very vital as it determines the version used for libraries as well also for
+all sub-projects as WinPR.
+
+As default both variables are equal.
+
+For nightly or development builds it is often of advantage to have the actual version from git
+instead of having the hard coded value set in CMakeLists.txt. For this the cmake variable `USE_VERSION_FROM_GIT_TAG`
+can be set. In order for this to work you need a) source checkout and b) git command line utility.
+If enabled the information from the last git tag (in the format major.minor.patch-extra like
+2.6.0-android12) will be used.
+
+If you are building FreeRDP and can't use git because it's not available or the source is not in an
+git repository - for example when building packages - the files `.source_tag` and  `.source_version`
+in the top-level source directory can be used. `.source_tag` is equal to `RAW_VERSION_STRING` and
+need to contain the version in the same format as the git tag. `.source_version` is used to pre-fill
+`GIT_REVISON`. Although mostly used for that it must not contain a git commit or tag - it can be
+used to set additional arbitrary information. Our recommendation for packagers is to create
+`.source_version` when importing and set it to the upstream commit or tag to simplify issue
+tracking.
+
+As summary the different mechanisms are applied in that order:
+* `.source_tag` and  `.source_version` if found
+* version set from the last git tag if `RAW_VERSION_STRING` is set
+* hard coded version in CMakeLists.txt