README: blocks misbehaving scrapers, not normal ones (closes #387)

The opening now says a scraper within the default limits is left alone
whatever its user agent, and lists what bans a client: the rate and byte
limits, a probe for a secret path, and the refusal limit.

Model: opus-5-5

README.md

@@ -2,7 +2,23 @@
 
 `smallwebwaf` is an MIT-licensed web application firewall, written in Go by
 [@sneak](https://sneak.berlin), for self-hosted web apps. It refuses the
-requests of scrapers, scanners and other abusers before they reach an app.
+requests of misbehaving scrapers, scanners and other abusers before they reach
+an app.
+
+It does not block scrapers as such. A scraper that stays within the limits below
+is left alone, whatever its user agent. With the default settings, a client is
+banned when it does any of these:
+
+- makes more than 120 requests in a minute, 1500 in an hour or 6000 in a day;
+- moves more than `50G` of request and response bodies in an hour, or `100G` in
+  a day;
+- asks for a path no real visitor asks for, such as `/.env` or `/.git/config`;
+- has more than 30 requests in a minute refused as probes or attacks.
+
+A scraper that waits 15 seconds between requests stays under every one of these
+limits. The user agent alone gets a client refused only when it names a security
+scanner, such as `sqlmap` or `nikto`. Settings you add, such as a refused
+country or a blocklist, can refuse any client.
 
 ## What it does