README: Add instructions for using external TBB

Author: hsbadr

README.md

@@ -10,6 +10,47 @@ The high level interface enables safe and robust parallel programming without di
 
 For additional documentation on using RcppParallel see the package website at http://rcppcore.github.io/RcppParallel/.
 
+### Intel TBB
+
+`RcppParallel` now supports the new interface of Intel TBB and allows using external library (e.g., with [oneTBB](https://github.com/oneapi-src/oneTBB) or [OneAPI](https://software.intel.com/content/www/us/en/develop/articles/intel-oneapi-toolkit-release-notes.html)), using `TBB_LIB` and `TBB_INC` environment variables. The updated TBB functionality are summarized [here](https://software.intel.com/content/www/us/en/develop/articles/tbb-revamp.html) and the release notes and backward compatibility are reported [here](https://software.intel.com/content/www/us/en/develop/articles/intel-oneapi-threading-building-blocks-release-notes.html).
+
+To build the development version of `RcppParallel` with [oneTBB](https://github.com/oneapi-src/oneTBB) or [OneAPI](https://software.intel.com/content/www/us/en/develop/articles/intel-oneapi-toolkit-release-notes.html):
+
+- Install [oneTBB](https://github.com/oneapi-src/oneTBB) or [OneAPI](https://software.intel.com/content/www/us/en/develop/articles/intel-oneapi-toolkit-release-notes.html).
+
+For example, installing [oneTBB](https://github.com/oneapi-src/oneTBB) on Linux 64-bit (`x86_64`) to `$HOME` directory (change if needed!):
+```
+    TBB_VERSION="2021.1.1"
+
+    wget https://github.com/oneapi-src/oneTBB/releases/download/v2021.1.1/oneapi-tbb-$TBB_VERSION-lin.tgz
+    tar zxvf oneapi-tbb-$TBB_VERSION-lin.tgz -C $HOME
+
+    export TBB="$HOME/oneapi-tbb-$TBB_VERSION"
+```
+
+- Set the TBB environment variables (specifically: `TBB` for the installation prefix, `TBB_INC` for the directory that includes the header files, and `TBB_LIB` for the libraries directory).
+
+For example, installing [oneTBB](https://github.com/oneapi-src/oneTBB) on Linux 64-bit (`x86_64`) to `$HOME` directory (change if needed!):
+```
+    source $TBB/env/vars.sh intel64
+
+    export TBB_INC="$TBB/include"
+    export TBB_LIB="$TBB/lib/intel64/gcc4.8"
+    #export LD_LIBRARY_PATH="$TBB_LIB:$LD_LIBRARY_PATH"
+
+    # Checks:
+    ls -lAh $TBB_INC
+    # should list `oneapi` and `tbb` directories that include the headers
+    ls -lAh $TBB_LIB
+    # should list the TBB libraries, including `libtbb.so`
+```
+
+- Build the development version of `RcppParallel`:
+```
+install.packages("remotes")
+remotes::install_github("hsbadr/RcppParallel@develop", force = TRUE)
+```
+
 ### License
 
 The RcppParallel package is made available under the [GPLv2](http://www.gnu.org/licenses/old-licenses/gpl-2.0.html).