Add a short documentation for migrating from v2 to v3

docs/migrate-v2-to-v3.md

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+# Migrating from v2 to v3
+
+If you want to migrate to v3, there are two basic approaches to do so.
+
+1. Use `catch_amalgamated.hpp` and `catch_amalgamated.cpp`.
+2. Build Catch2 as a proper (static) library, and move to piecewise headers
+
+Doing 1 means downloading the [amalgamated header](/extras/catch_amalgamated.hpp)
+and the [amalgamated sources](/extras/catch_amalgamated.cpp) from `extras`,
+dropping them into your test project, and rewriting your includes from
+`<catch2/catch.hpp>` to `"catch_amalgamated.hpp"` (or something similar,
+based on how you set up your paths).
+
+The disadvantage of using this approach are increased compilation times,
+at least compared to the second approach, but it does let you avoid
+dealing with consuming libraries in your build system of choice.
+
+
+However, we recommend doing 2, and taking extra time to migrate to v3
+properly. This lets you reap the benefits of significantly improved
+compilation times in the v3 version. The basic steps to do se are:
+
+1. Change your CMakeLists.txt to link against `Catch2WithMain` target if
+you use Catch2's default main. (If you do not, keep linking against
+the `Catch2` target.)
+2. Delete TU with `CATCH_CONFIG_RUNNER` or `CATCH_CONFIG_MAIN` defined,
+as it is no longer needed.
+3. Change `#include <catch2/catch.hpp>` to `#include <catch2/catch_all.hpp>`
+4. Check that everything compiles. You might have to modify namespaces,
+or perform some other changes (see the
+[Things that can break during porting](#things-that-can-break-during-porting)
+section for the most common things).
+5. Start migrating your test TUs from including `<catch2/catch_all.hpp>`
+to piecemeal includes. You will likely want to start by including
+`<catch2/catch_test_macros.hpp>`, and then go from there. (see
+[other notes](#other-notes) for further ideas)
+
+## Other notes
+* The main test include is now `<catch2/catch_test_macros.hpp>`
+* Big "subparts" like Matchers, or Generators, have their own folder, and
+also their own "big header", so if you just want to include all matchers,
+you can include `<catch2/matchers/catch_matchers_all.hpp>`,
+or `<catch2/generators/catch_generators_all.hpp>`
+
+
+## Things that can break during porting
+* The namespace on Matchers were cleaned up, they are no longer first declared
+deep within an internal namespace and then brought up. All Matchers now live
+in `Catch::Matchers`.
+* The reporter interfaces changed in a breaking manner. If you wrote custom
+reporter or listener, you might need to modify them a bit.
+
+
+---
+
+[Home](Readme.md#top)