add README

README.md

+# `framework-arduinoteensy-cxx20`
+A fork of the PlatformIO ArduinoTeensy framework compatible with GCC 10 & C++20.
+
+# Usage
+
+## Prerequisites
+
+In order to build with this framework, you'll need to clone a few repos.
+
+### Clone framework and dependencies
+
+First, clone this repository:
+
+```sh
+git clone https://github.com/thevisus/framework-arduinoteensy-cxx20.git
+```
+
+This framework is compatible with the GNU ARM Embedded Toolchain v10.2.1. A
+corresponding [PlatformIO
+toolchain](https://github.com/thevisus/toolchain-gcc-arm-embedded) is therefore
+necessary:
+
+```sh
+git clone https://github.com/thevisus/toolchain-gcc-arm-embedded.git
+```
+
+Finally, a PlatformIO "platform" is needed to drive the build:
+
+```sh
+git clone https://github.com/thevisus/teensy-gcc10.git
+```
+
+Each of these needs to be installed locally via PlatformIO. The `pio` command or
+similar management software should be able to do this. The simplest method, on
+Linux/macOS is to symlink the projects into the PlatformIO user directory:
+
+```sh
+ln -s /path/to/framework-arduinoteensy-cxx20 $HOME/.platformio/packages
+ln -s /path/to/toolchain-gcc-arm-embedded $HOME/.platformio/packages
+ln -s /path/to/teensy-gcc10 $HOME/.platformio/platforms
+```
+
+You may check the full platform's availability using `pio`:
+
+```sh
+pio platform show teensy-gcc10
+```
+
+## Configuring a project
+
+In your project's `platform.ini`, adjust your environment settings similar to
+the following:
+
+```ini
+[platformio]
+default_envs = teensy41
+
+[env:teensy41]
+platform = teensy-gcc10
+platform_packages = toolchain-gcc-arm-embedded
+board = teensy41
+framework = arduino
+```
+
+## Building a project
+
+If everything is setup correctly, you may build your project per usual.
+
+```sh
+# compile
+pio run
+# upload
+pio run --target upload
+```
+
+### Example Makefile
+
+Driving the pio process via `make` can be accomplished with a simple Makefile.
+(NB: this version assumes `make` will be invoked from within `vim`; if that's
+not the case, remove the `-c vim` portions.)
+
+```Makefile
+all:
+	pio -c vim run
+
+upload:
+	pio -c vim run --target upload
+
+clean:
+	pio -c vim run --target clean
+
+program:
+	pio -c vim run --target program
+
+uploadfs:
+	pio -c vim run --target uploadfs
+
+update:
+	pio -c vim update
+```
+
+# Upstream
+The code in `cores` is up-to-date as of late 2020 from the original
+https://github.com/PaulStoffregen/cores repo, combined with the libraries and
+tools found in PlatformIO's original framework-arduinoteensy. It's a bit of a
+mish-mash, but is fairly up-to-date and functional (tested and run in live
+systems for several months now).
+
+However, due to the amalgamation of repos and a variety of other reasons, the
+`cores` code will only be minimally updated for critical fixes, if at all.
+Instead, a leaner version of the 4.1 core, intended to be built with the
+[dds](https://github.com/vector-of-bool/dds) toolchain is actively maintained at
+my personal Gitea: https://git.thevis.us/visus/teensy-core-4.1
+
+# License
+```
+Teensyduino Core Library
+http://www.pjrc.com/teensy/
+Copyright (c) 2017 PJRC.COM, LLC.
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+1. The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+2. If the Software is incorporated into a build system that allows
+selection among a list of target devices, hen similar target
+devices manufactured by PJRC.COM must be included in the list of
+target devices and selectable in the same manner.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+```
+