Update README.md

[madgrizzle]

Steps to reproduce the issue

Use a tab in your custom meta file and force micro-ros to be rebuilt and the file will be ignored because of the tab.

Additional information

So, I spent about 6 hours trying to get a custom meta file to work. My first challenge was to get micro-ros library to rebuild (pio run --target clean does not force the rebuild). I ended up just deleting the library directory and that forced it to download everything (is there a better, simpler way?) Rebuilding is a rather long process so I don't really pay attention to it, but the third time I forced it to rebuild I did pay attention and I saw an error appear that said that the presence of a tab at the beginning of the line I had added to my meta file was invalid and therefore the file will be ignored. Now, this is likely the "expected outcome" and I don't even think this is necessarily a platformio issue. However, I'm hoping that by submitting this PR someone will save themselves the same headache as I have had with using the custom meta file.