README: add/explain SetMaxOpenConns(1)

[stapelberg]

Concurrent transactions are unsupported by txdb.

related to issue #69

[flimzy]

Thanks for the PR. My concern with this is that I think it's overly prescriptive. It's not always adviseable to set max connections to 1. It depends a lot on your application. I think some additional explanation is warranted. I'll see if I can find some time to do that, if you don't get to it first.

[stapelberg]

Thanks for the PR. My concern with this is that I think it's overly prescriptive. It's not always adviseable to set max connections to 1. It depends a lot on your application. I think some additional explanation is warranted. I'll see if I can find some time to do that, if you don't get to it first.

I’m not sure why setting max connections to 1 isn’t a good default. Over in issue #69 you mentioned that this could lead to deadlocks in certain applications, but I’m not sure under which conditions exactly. (And probably a deadlock is easier to debug than the current flakyness.)

Feel free to modify the PR directly, or merge and adjust as you see fit.