Add C++ Boost library example

While Linux and Windows have Boost preinstalled, on macOS, it is not.
As downloading and building boost takes a lot of time, caching the
resulting install dir can result in considerable resource saving.
The shown example was tested here:
https://github.com/Morgan-Stanley/binlog/actions/runs/50704053
Successful cache hit:
https://github.com/Morgan-Stanley/binlog/runs/493249216

README.md

@@ -60,6 +60,7 @@ Every programming language and framework has its own way of caching.
 
 See [Examples](examples.md) for a list of `actions/cache` implementations for use with:
 
+- [C++ - Boost](./examples.md#c---boost)
 - [C# - Nuget](./examples.md#c---nuget)
 - [Elixir - Mix](./examples.md#elixir---mix)
 - [Go - Modules](./examples.md#go---modules)

examples.md

@@ -1,6 +1,7 @@
 # Examples
 
 - [Examples](#examples)
+  - [C++ - Boost](#c---boost)
   - [C# - NuGet](#c---nuget)
   - [Elixir - Mix](#elixir---mix)
   - [Go - Modules](#go---modules)
@@ -28,6 +29,40 @@
   - [Swift, Objective-C - CocoaPods](#swift-objective-c---cocoapods)
   - [Swift - Swift Package Manager](#swift---swift-package-manager)
 
+## C++ -Boost
+
+Downloading, building and installing some boost libraries:
+
+```yaml
+- name: Cache Boost
+  id: cache-boost
+  uses: actions/cache@v1
+  with:
+    path: boost_1_72_0/installdir
+    key: ${{ runner.os }}-boost_1_72_0-fs-system-test
+- name: Install Boost
+  if: steps.cache-boost.outputs.cache-hit != 'true'
+  run: |
+    wget https://dl.bintray.com/boostorg/release/1.72.0/source/boost_1_72_0.tar.gz
+    tar -zxf boost_1_72_0.tar.gz
+    cd boost_1_72_0
+    mkdir -p installdir
+    ./bootstrap.sh --prefix=./installdir --with-libraries=filesystem,system,test
+    ./b2 install
+```
+
+In this example, only a subset is built, to speed up compilation and reduce footprint.
+See `./bootstrap.sh --show-libraries` for the complete set of libraries.
+
+To be used the just installed boost in conjuction with CMake:
+
+```yaml
+cd boost_1_72_0/installdir
+BOOST_ROOT=$(pwd)
+cd ../../
+cmake -DBOOST_ROOT=${BOOST_ROOT} ....
+```
+
 ## C# - NuGet
 Using [NuGet lock files](https://docs.microsoft.com/nuget/consume-packages/package-references-in-project-files#locking-dependencies):