skip loading readme files if it won't fit in the DB

[QuietMisdreavus]

Some crates have excessively long README files, that are too big for the database column. This PR changes the README loading function to discard a README if it wouldn't fit in the database, using a similar check that the read_rust_doc function uses.

[onur]

Interesting I thought 50KB should be enough for README files (I have no idea why I assumed first TBH). Maybe instead of ignoring this files, we can increase storage size in the database.

[QuietMisdreavus]

That would be a schema change; should we wait for #305 to land first?

[QuietMisdreavus]

Actually, i'm not sure increasing the size of the column is a good long-term fix - someone will wind up making an even longer README than what we decide on, whether accidentally or deliberately. We should have a way to properly deal with those crates. Right now they just sit in the queue at maximum attempts and don't even register as failed.

[QuietMisdreavus]

I'm going to go ahead and merge this in, since it echoes the code in read_rust_doc farther down the file. #305 is merged now, so we can easily increase the storage size for READMEs and doc headers in the future, but for now, i'd rather put this guard in. This will allow more crates to be loaded in.