README.md: initial version

Signed-off-by: akarin <i@akarin.info>

README.md

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+VapourSynth api3-to-api4 brdige
+-------------------------------
+
+# Introduction
+
+Recent VapourSynth releases (R55+) introduced a new API version (api4) that is
+not compatible with the previous plugin API (api3). Even though newer VS versions
+can load plugins using either api3 or api4, older VS versions are stuck with
+api3 plugins. This poses a dilemma for (video) plugin authors:
+ - either you abandon users still stuck with older api3 VS versions by migrating to api4
+ - or, you maintain two copies (branches) of essentially the same code.
+
+The best course of action, IMHO, is to stick with api3 to support both worlds
+without maintaining two copies of essentially the same code.
+
+As anyone who has migrated video plugin from api3 to api4 would testify, there
+aren't that many differences between the two APIs. One might ask: is it possible
+to support both both API versions without duplicating code?
+
+The answer is yes, and this project is one solution that should help with this
+situation.
+
+# Usage for End Users
+
+For end users of VS plugins, this project can be used to load api4 *vdieo* plugins
+in api3 VS. (api3 VS does not have audio support, so obviously this project won't
+change that.)
+
+Suppose you are stuck with VS R54, but you want to use a *video* plugin `filter.dll`
+that only has api4 support. You can download the released `api3.dll` and save it
+as `filter.api3.dll` along side with `filter.dll` into VS `plugins` directory.
+
+Note for Unix users: as this process involves making multiple copies of `api3.so`
+if you have multiple api4 plugins to wrap, in order to save disk space, please
+use *hard* links instead of symbolic links.
+
+# Usage for Plugin Developers
+
+For plugin developers, this project can help you support both api3 and api4 without
+any code duplication. Just migrate to api4 and include api3.cc in your project. And
+the rest should be automatically handled.