Add BUILD.md doc

BUILD.md

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+> In order to create an deterministic build, we're using docker.
+
+### 1. Install Docker
+
+For macOS, download it at https://hub.docker.com/editions/community/docker-ce-desktop-mac
+
+### 2. Prepare repo
+
+```shell
+# Clone repo and submodules
+git clone git@github.com:ExodusMovement/mymonero-core-js.git --recursive
+cd mymonero-core-js
+
+# Remove the existing files, we'll build them in the next section
+rm monero_utils/MyMoneroCoreCpp_*
+
+# Prepare boost source code
+curl -LO https://dl.bintray.com/boostorg/release/1.69.0/source/boost_1_69_0.tar.gz
+mkdir -p contrib/boost-sdk
+tar zxf boost_1_69_0.tar.gz -C contrib/boost-sdk --strip-components=1
+```
+
+### 3. Build emscripten
+
+```shell
+# Build boost emscripten
+docker run -it -v $(pwd):/app quay.io/exodusmovement/emscripten ./bin/build-boost-emscripten.sh
+
+# Build MyMonero emscripten
+docker run -it -v $(pwd):/app quay.io/exodusmovement/emscripten ./bin/archive-emcpp.sh
+
+# If you get '#error Including <emscripten/bind.h> requires building with -std=c++11 or newer!' error, re-run:
+
+docker run -it -v $(pwd):/app quay.io/exodusmovement/emscripten ./bin/archive-emcpp.sh
+
+# Create monero_utils/MyMoneroCoreCpp_* files, they should be same as the ones in repo.
+```
+
+# Other Notes
+
+The `quay.io/exodusmovement/emscripten` image was built by Quay.io
+See instructions at https://github.com/ExodusMovement/docker-emscripten