Add wasm build instructions to documentation (#196)

* Add wasm build instructions to documentation

* Update wasm instructions to match readme.md

Author: mcbarton

docs/source/InstallationAndUsage.rst

@@ -1,7 +1,7 @@
 Installation And Usage
 --------------------
 
-Installation from source
+Installation from source (non wasm build instructions)
 ========================
 
 To ensure that the installation works, it is preferable to install `xeus-cpp` in a
@@ -42,6 +42,68 @@ with a custom installation prefix if need be)
     -D CMAKE_INSTALL_LIBDIR=lib
     make && make install
 
+Installation within a mamba environment (wasm build instructions)
+========================
+
+These instructions will assume you have cmake installed on your system. First clone the repository, and move into that directory
+.. code-block:: bash
+    git clone --depth=1 https://github.com/compiler-research/xeus-cpp.git
+    cd ./xeus-cpp
+
+
+You'll now want to make sure you're using emsdk version "3.1.45" and activate it. You can get this by executing the following
+
+.. code-block:: bash
+    cd $HOME
+    git clone https://github.com/emscripten-core/emsdk.git
+    cd emsdk
+    ./emsdk install 3.1.45
+    ./emsdk activate 3.1.45
+    source $HOME/emsdk/emsdk_env.sh
+
+
+You are now in a position to build the xeus-cpp kernel. You build it by executing the following
+
+.. code-block:: bash
+    micromamba create -f environment-wasm-host.yml --platform=emscripten-wasm32
+    mkdir build
+    pushd build
+    export PREFIX=$MAMBA_ROOT_PREFIX/envs/xeus-cpp-wasm-host 
+    export CMAKE_PREFIX_PATH=$PREFIX
+    export CMAKE_SYSTEM_PREFIX_PATH=$PREFIX
+    export SYSROOT_PATH=$HOME/emsdk/upstream/emscripten/cache/sysroot
+    emcmake cmake \
+            -DCMAKE_BUILD_TYPE=Release                        \
+            -DCMAKE_PREFIX_PATH=$PREFIX                       \
+            -DCMAKE_INSTALL_PREFIX=$PREFIX                    \
+            -DXEUS_CPP_EMSCRIPTEN_WASM_BUILD=ON               \
+            -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ON            \
+            -DSYSROOT_PATH=$SYSROOT_PATH                      \
+            ..
+    emmake make install
+
+
+To build Jupyter Lite with this kernel without creating a website you can execute the following
+
+.. code-block:: bash
+    micromamba create -n xeus-lite-host jupyterlite-core
+    micromamba activate xeus-lite-host
+    python -m pip install jupyterlite-xeus
+    jupyter lite build --XeusAddon.prefix=$PREFIX
+
+We now need to shift necessary files like `xcpp.data` which contains the binary representation of the file(s)
+we want to include in our application. As of now this would contain all important files like Standard Headers,
+Libraries etc coming out of emscripten's sysroot. Assuming we are still inside build we should do the following
+
+.. code-block:: bash
+    cp $PREFIX/bin/xcpp.data _output/extensions/@jupyterlite/xeus/static
+    cp $PREFIX/lib/libclangCppInterOp.so _output/extensions/@jupyterlite/xeus/static
+
+Once the Jupyter Lite site has built you can test the website locally by executing
+
+.. code-block:: bash
+    jupyter lite serve --XeusAddon.prefix=$PREFIX
+
 Installing from conda-forge
 ===========================
 
@@ -51,4 +113,4 @@ If you have conda installed then you can install xeus-cpp using the follwing com
 
     conda install conda-forge::xeus-cpp
 
-Xeus-cpp is available for Linux, MacOS and Windows.
+Xeus-cpp is available for Linux, MacOS and Windows.