docs: add postcard/postcard-rpc to README.md #31

[djessup]

Adds postcard + postcard-rpc to the (de)serialisation wip list as discussed in #31

Category

Is this PR a:

• New Not Yet Awesome item?
• A WIP project addressing an open item?
• Removing a Not Yet Awesome item?

WIP Project checklist

• Is there a link to the source project on GitHub/GitLab/etc?
• Is the project open source?
• Is it clear if contributions are welcome?
• Is the project relevant to the TODO item it addresses?
• Is the project more than just an empty repo or a placeholder (e.g. just a "hello world" bin.rs/lib.rs)?

[eldruin]

Thank you for your addition. However, we are in the process of archiving this repository. See #32.
Apologies for that.