add README to explain setting the wifi network/pass, also remove .gitignore line as that doesn't work properly, the README gives a better alternative.

src/secrets/README.md

@@ -0,0 +1,17 @@
+# Secret Stuff!
+
+To set your wifi network and password put them into the relevant files but
+without a trailing newling. I.e.
+
+```
+echo -n "my-wifi-network" > src/secrets/wifi-network
+echo -n "some password" > src/secrets/wifi-password
+```
+
+This command can then be used to ensure changes don't get checked in:
+
+```
+git update-index --assume-unchanged src/secrets/wifi-network
+git update-index --assume-unchanged src/secrets/wifi-password
+```
+