Add install and setup notes

Signed-off-by: Dominik Schulz <[email protected]>

README.md

@@ -1,11 +1,34 @@
 # gopass-hibp
+
 Gopass haveibeenpwnd.com integration
 
 ## Installation
 
 ### Fedora
+
 To install gopass-hibp in Fedora you can do:
 
 ```bash
 sudo dnf install gopass-hibp
 ```
+
+### From Source
+
+```bash
+go install github.com/gopasspw/gopass-hibp@latest
+```
+
+## Setup
+
+If you want to use the offline mode (`dump`) you need to obtain a HIBP dump (SHA1, ordered by hash).
+You can use one of the [official sources](https://haveibeenpwned.com/Passwords) but they haven't been
+updated in a while and it [seems](https://www.troyhunt.com/downloading-pwned-passwords-hashes-with-the-hibp-downloader/) like they won't receive further updates.
+
+Instead prefer the built-in downloader. It does use the same approach as the official .NET tool.
+
+```bash
+gopass-hibp download --output /some/folder/with/40G/dump.txt.gz
+```
+
+The data will be downloaded into a million chunks first and then assembled to a large file later.
+The output file will be around 18GB in size. During assembly of the chunks it will use twice that space for a short time.