Add instructions for adding support for new MSVS version

Try to collect all the information in a single place to make things
easier for the next update.

docs/contributing/how-to-add-new-msvs-version.md

@@ -0,0 +1,58 @@
+How to add support for a new MSVS version
+=========================================
+
+MSVS versions
+-------------
+
+This is confusing because there are several of them:
+
+- Marketing IDE/product version, which is the year of the release, e.g. 2019.
+- Actual internal IDE version, visible in `Help|About` dialog, for example,
+  e.g. 16.1.4.
+- Compiler version, visible in `cl /?` output, e.g. 19.21.27702.2. This version
+  also corresponds to the value of the predefined `_MSC_VER` symbol.
+- Toolset version: this is similar to the compiler version, but uses different
+  format, e.g. it's 14x for compiler version 19, with `x` being 0, 1 or 2
+  depending on the minor compiler version (00, 14 or 21).
+
+Moreover, historically, different IDE versions corresponded to different major
+compiler versions, however this hasn't been the case any more for the last few
+releases (2015, 2017, 2019 or 14, 15, 16 IDE versions) that all use the same
+major compiler version (19) as they are ABI-compatible. Also, historically,
+each compiler used its own toolset, but these last few versions can each use
+any of 140, 141, 142 toolset.
+
+wxWidgets uses the IDE version for the project/solution files (but notice that
+project files are the same for all MSVS 201x versions) and the toolset version
+for the names of build directories.
+
+
+TODO list
+---------
+
+Here is a list of things that need to be done when adding support for the next
+MSVS version:
+
+- If the major compiler version changed, add a new `__VISUALCxx__` symbol to
+  `include/wx/compiler.h` (`xx` here are the first 2 digits of the toolset
+  version, i.e. basically correspond to the major compiler version, except
+  that 13 was skipped for superstitious reasons, see the comment in that file).
+- Add `wxCOMPILER_PREFIX` definition for the new version to
+  `include/msvc/wx/setup.h`.
+- Add the new solution file `build/msw/wx_vcNN.sln` (where `NN` is the internal
+  IDE version), typically by opening an existing solution in the new IDE and
+  letting it to update it.
+- Discard changes to the project files that were done by the IDE during the
+  update and instead update `build/msw/wx_config.props` and add a new condition
+  checking `VisualStudioVersion` value for choosing the value of
+  `PlatformToolset`.
+- Update the lists of supported compiler versions in the different
+  documentation files (search for "Microsoft Visual C++" to find the places
+  that need updating):
+  - `README.md`
+  - `docs/doxygen/mainpages/introduction.h`
+  - `docs/msw/install.md`
+  - `docs/release.md`
+- Update `build/tools/msvs/officialbuild.bat` to support the new toolset version
+  and also update `build/tools/msvs/buildall.bat` to call it with this new version
+  (these files are used to build the official binaries for the releases).