First draft of Build.md build documentation

Author: samuelpilz

docs/Build.md

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+## Build system
+
+`haskell-ide-engine` is built using the library `shake`. The build descriptions are defined in the file `install.hs`.
+
+### Design decisions
+
+The design of the build system follows 2 main goals:
+
+* works same on every platform
+* has minimal run-time dependencies: `stack` and `git`
+* is completely functional right after simple `git clone`
+* one-stop-shop for all building
+
+Previously, `haskell-ide-engine` was built using a `Makefile` on Unix systems and a `PowerShell` script on Windows. By replacing both scripts by a Haskell-based solution, the scripts can be can be replaced by a single script.
+
+### Tradeoffs
+
+#### `shake.yaml`
+
+A `shake.yaml` is required for executing the `install.hs` file.
+
+* It contains the required version of `shake`.
+* In contrast to the other `*.yaml` it does not contain the submodules, which is necessary for `stack` to work even before the submodules have been initialized.
+
+It is necessary to update the `resolver` field of the `shake.yaml` if the .
+
+#### `install.hs` installs a GHC
+
+Before the code in `install.hs` can be executed, `stack` installs a `GHC`, depending on the `resolver` field in `shake.yaml`. This is a necessary if the `install.hs` should be totally functional right after a fresh `git clone` without further configuration.
+
+This may lead to an extra `GHC` to be installed by `stack` if not all versions of `haskell-ide-engine` are installed.