README: add warning about internet access

gitsign differs from GPG in that it requires an internet access on every
commit. This breaks the git model of working offline. So, probably worth
calling out.

README.md

@@ -32,19 +32,27 @@ Single Repository:
 
 ```sh
 cd /path/to/my/repository
-git config --local commit.gpgsign true  # Sign all commits
-git config --local tag.gpgsign true  # Sign all tags
 git config --local gpg.x509.program gitsign  # Use gitsign for signing
 git config --local gpg.format x509  # gitsign expects x509 args
+
+# Warning: Setting this will make git commit/tag reliant on internet.
+# Alternatively, don't use these settings and add the -S flag instead.
+git config --local commit.gpgsign true  # Sign all commits
+git config --local tag.gpgsign true  # Sign all tags
+
 ```
 
 All respositories:
 
 ```sh
-git config --global commit.gpgsign true  # Sign all commits
-git config --global tag.gpgsign true  # Sign all tags
 git config --global gpg.x509.program gitsign  # Use gitsign for signing
 git config --global gpg.format x509  # gitsign expects x509 args
+
+
+# Warning: Setting this will make git commit/tag reliant on internet.
+# Alternatively, don't use these settings and add the -S flag instead.
+git config --global commit.gpgsign true  # Sign all commits
+git config --global tag.gpgsign true  # Sign all tags
 ```
 
 To learn more about these options, see