Add the basic developer's documentation for the Wasm port

docs/DEVELOPMENT.md

@@ -43,6 +43,10 @@ If you would like to build and run this project on PlayStation Vita please follo
 
 If you would like to build and run this project on Nintendo Switch please follow the instructions on [**this page**](README_switch.md).
 
+### Emscripten (Wasm)
+
+If you would like to run this project in a web browser please follow the instructions on [**this page**](README_emscripten.md).
+
 ### Build with CMake
 
 If you would like to build the project using CMake please follow the instructions on [**this page**](README_cmake.md).

docs/README_emscripten.md

@@ -0,0 +1,68 @@
+# Emscripten (Wasm) port of [**fheroes2**](README.md) project
+
+## Current status of this port
+
+Please note that this port is currently just an **experiment**. It may experience various problems, crash, or even not work at all in a particular browser.
+
+## Building
+
+### Prerequisites
+
+* emsdk 4.0 or later
+* GNU gettext
+
+### Building using Docker
+
+To save time, you can use a ready-made Docker image:
+
+```sh
+docker run --rm -v "$(pwd):/src" emscripten/emsdk:latest sh -c "apt-get -y update; apt-get -y install gettext; emmake make -f Makefile.emscripten"
+```
+
+### Building without Docker
+
+If you do not want to use Docker, then you will need to install all the prerequisites manually using your platform's package manager, and then run the following command:
+
+```sh
+emmake make -f Makefile.emscripten
+```
+
+### Building with additional parameters
+
+If you want to specify some additional ad hoc build parameters, you can use the appropriate environment variables for this, for example:
+
+```sh
+FHEROES2_WITH_DEBUG=ON LDFLAGS="-sMODULARIZE -sEXPORTED_RUNTIME_METHODS=run -sEXPORT_NAME=fheroes2" emmake make -f Makefile.emscripten
+```
+
+will build fheroes2 in debug mode with additional parameters to create a module.
+
+## Running the Wasm port on a web server
+
+### Configuring the web server
+
+Emscripten uses web workers along with `SharedArrayBuffer` to support multithreading. `SharedArrayBuffer` usage is usually protected by Cross Origin Opener Policy (COOP)
+and Cross Origin Embedder Policy (COEP) HTTP headers, and the web server should send these headers as part of its response to a request for resources related to the Wasm
+build:
+
+```text
+Cross-Origin-Opener-Policy: same-origin
+Cross-Origin-Embedder-Policy: require-corp
+```
+
+Please see the following link for details:
+
+<https://web.dev/coop-coep>
+
+### Copying necessary files
+
+After the build is completed, copy the following files to the appropriate directory on your website:
+
+```text
+fheroes2.data
+fheroes2.js
+fheroes2.wasm
+fheroes2.wasm.map (if fheroes2 has been built in debug mode)
+```
+
+If you want to use the stock launcher (pretty basic at the moment), then also copy all the files from `files/emscripten` to the same directory.