add READMEs for cert/vpn subdirs

cert_instructions/README.md

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+## How to Install to your Bubble Certificate
+
+Installing your Bubble Certificate depends on what system or device you're using.
+
+### [Windows](windows_cert.md)
+
+### [Mac OS X](macosx_cert.md)
+
+### [Linux](linux_cert.md)
+
+### [Apple iOS](ios_cert.md)
+
+### [Android](android_cert.md)

vpn_instructions/README.md

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+## How to Connect to your Bubble VPN
+
+Connecting to your Bubble VPN depends on what system or device you're using.
+
+### [Windows](windows_vpn.md)
+
+### [Mac OS X](macosx_vpn.md)
+
+### [Linux](linux_vpn.md)
+
+### [Apple iOS](ios_vpn.md)
+
+### [Android](android_vpn.md)