Add MathiasKoch to maintainer list

[MathiasKoch]

As with Ryan in #10, i was approached by @eldruin about helping out with supporting embedded-nal, which i of course can spare some time for.

This PR adds myself to the list of maintainers in REC.

Of relevant stuff about myself, i can say that we are running embedded rust IoT devices in production at my workplace, developed by me.

We have a stack making use of a larger number of projects we have OS'ed, a lot of which is still WIP, including:

• https://github.com/BlackbirdHQ/atat/
• https://github.com/BlackbirdHQ/ublox-cellular-rs/
• https://github.com/BlackbirdHQ/ublox-short-range-rs/
• https://github.com/BlackbirdHQ/mqttrust/
• https://github.com/BlackbirdHQ/rustot/
• https://github.com/BlackbirdHQ/defmt-persist
• https://github.com/BlackbirdHQ/at-cryptoauth-rs

Other than that i am also an active contributor and maintainer of the HAL we are using: https://github.com/stm32-rs/stm32l4xx-hal

[eldruin]

@MathiasKoch Could you rebase this?

[MathiasKoch]

@eldruin Done :+1:

[eldruin]

And consensus again. Thanks everyone and welcome @MathiasKoch!