Link Git-specific notes from the main README

Rename BuildGit.txt to README-GIT.md, which is a more standard name, and
a better format for viewing this file online, e.g. from GitHub Web UI,
and link it from the main README to make it easier to find.

BuildGit.txt

@@ -1,17 +0,0 @@
-------------------------------------------------------------------------
-                How to build the sources from git
-------------------------------------------------------------------------
-
-When cloning the repository, use --recurse-submodules option. If you had
-already cloned it without it, run "git submodule update --init" to also
-get all 3rd party libraries code.
-
-Before building from git sources under Windows, you need to copy the
-file include/wx/msw/setup0.h to include/wx/msw/setup.h. This is
-necessary in order to allow having local modifications to the latter
-file without showing it as modified, as it is not under version
-control.
-
-After doing this, please proceed with the build as with any official
-release, i.e. follow the instructions in the install.txt file in
-docs/gtk, docs/msw and so on.

README-GIT.md

@@ -0,0 +1,17 @@
+Building from Git Sources
+-------------------------
+
+Please use `--recurse-submodules` option with the initial `git clone`. If you
+had already cloned the repository without this option, please run `git
+submodule update --init` to also get all 3rd party libraries code.
+
+Before building from Git sources under Windows, you need to copy the
+file `include/wx/msw/setup0.h` to `include/wx/msw/setup.h`. This is
+necessary in order to allow having local modifications to the latter
+file without showing it as modified, as it is not under version
+control.
+
+After doing this, please proceed with the build as with any official
+release, i.e. follow the instructions in the port-specific files, e.g.
+[wxMSW](docs/msw/install.txt), [wxGTK](docs/gtk/install.txt),
+[wxOSX](docs/osx/install.txt) and so on.

README.md

@@ -45,6 +45,16 @@ is a modified version of LGPL explicitly allowing not distributing the sources
 of an application using the library even in the case of static linking.
 
 
+Building
+--------
+
+For building the library, please see platform-specific documentation under
+`docs/<port>` directory.
+
+If you're building the sources checked out from Git, and not from a released
+versions, please see these additional [Git-specific notes](README-GIT.md).
+
+
 Further information
 -------------------