Add linux build instructions

{{cookiecutter.project_name}}/BUILD_LINUX.md

@@ -0,0 +1,47 @@
+# Build and Package {{cookiecutter.project_name}}
+
+This document summarizes how to build and package {{cookiecutter.project_name}} on Linux.
+
+{{cookiecutter.project_name}} is a custom Slicer application. Reading the [3D Slicer Developer Documentation](https://slicer.readthedocs.io/en/latest/developer_guide/index.html) may help answer additional questions.
+
+The initial source files were created using [KitwareMedical/SlicerCustomAppTemplate](https://github.com/KitwareMedical/SlicerCustomAppTemplate).
+
+## Clone
+
+```sh
+git clone https://github.com/{{cookiecutter.github_organization}}/{{cookiecutter.github_project}}.git
+```
+
+## Prerequisites
+
+Install the prerequisites as described in the [Slicer documentation for building on Linux](https://slicer.readthedocs.io/en/latest/developer_guide/build_instructions/linux.html). This includes development tools, support libraries, and Qt.
+
+## Build
+
+Note: The build process can take hours.
+
+Build:
+
+```sh
+cmake \
+  -DCMAKE_BUILD_TYPE:STRING=Release \
+  -DQt5_DIR:PATH=/opt/qt/5.15.2/gcc_64/lib/cmake/Qt5 \
+  -S {{cookiecutter.github_project}} \
+  -B {{cookiecutter.github_project}}-SuperBuild-Release
+cd {{cookiecutter.github_project}}-SuperBuild-Release
+make -j<N>
+```
+
+where `<N>` is the number of parallel builds. As a rule of thumb, many use the `number of CPU threads - 1` as the number of parallel builds.
+On Ubuntu 20.04, the default Qt5 packages are too old and so the Slicer documentation linked above should have suggested a method of installing Qt 5.15.2;
+if you installed it to `/opt/qt`, for example, then an extra option like `-DQt5_DIR:PATH=/opt/qt/5.15.2/gcc_64/lib/cmake/Qt5` would be needed in the `cmake` command above.
+
+Once the application is built, there will be an _inner build_ inside the _superbuild_ folder, located at `{{cookiecutter.app_name}}-SuperBuild-Release/Slicer-build`. The application executable is contained in this _inner build_ folder.
+
+## Package
+
+From the _inner build_ folder:
+
+```sh
+make package
+```

{{cookiecutter.project_name}}/BUILD.md → {{cookiecutter.project_name}}/BUILD_WINDOWS.md

@@ -16,9 +16,6 @@ The initial source files were created using [KitwareMedical/SlicerCustomAppTempl
 
   - Setup [your git username](https://help.github.com/articles/setting-your-username-in-git) and [your git email](https://help.github.com/articles/setting-your-email-in-git).
 
-  - If not already done, email `FirstName LastName <firstname.lastname@{{cookiecutter.github_organization}}.com>` to be granted access to
-    the [{{cookiecutter.github_organization}}/{{cookiecutter.github_project}}](https://github.com/{{cookiecutter.github_organization}}/{{cookiecutter.github_project}}) repository.
-
 ## Checkout
 
 1. Start `Git Bash`
@@ -35,7 +32,7 @@ Note: use short source and build directory names to avoid the [maximum path leng
 
 ## Build
 
-Note: The build process will take approximately 3 hours.
+Note: The build process can take hours.
 
 <b>Option 1: CMake GUI and Visual Studio (Recommended)</b>
 

{{cookiecutter.project_name}}/README.md

@@ -16,6 +16,7 @@ _List key features_
 ## Development
 
 - [Contributing](CONTRIBUTING.md)
-- [Building](BUILD.md)
+- [Building on Windows](BUILD_WINDOWS.md)
+- [Building on Linux](BUILD_LINUX.md)
 
 ![{{cookiecutter.project_name}} by {{cookiecutter.org_name}}](Applications/{{cookiecutter.app_name}}App/Resources/Images/LogoFull.png?raw=true)