README.md: add a brief note on using big-endian types

[bentorkington]

In case this helps with #33 You might prefer a fully fleshed-out example, but this should at least help someone searching the README for 'big endian' to find out how to use it quickly

[coveralls]

Coverage: 100.0%. Remained the same when pulling 1be094e488de1e09d8934f411958702257eac555 on bentorkington:bigendian-note into 326345b0c87d2a98116b22a236ea6623180ffbb1 on pabigot:main.

[bentorkington]

The build errored. This is a change from the previous build, which passed.

node: /lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.27' not found (required by node)
node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.25' not found (required by node)
node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by node)
node: /lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.27' not found (required by node)
node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.25' not found (required by node)
node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by node)
The command "npm config set progress false" failed and exited with 1 during

🤷‍♂️ wasn't me, all I did was add a few lines to the README.md