Add help doc that documents fixing database corruption

doc/help.md

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+# How To Fix Various Problems
+
+This document will document how to recover and fix various issues that users may run into.
+
+## Database corruption
+
+The problem: You see an error like
+
+```
+EXCEPTION: 15dbwrapper_error
+Database corrupted
+```
+
+This means your blockchain data on disk is corrupted. This is not the same as wallet corruption.
+The fix to this is to do a fresh sync using the same wallet.dat file. You need to find
+where this data lives on your computer and then move some files around, then restart the wallet.
+
+### On Linux
+
+If you are on Linux, your wallet lives at `~/.hush/HUSH3/wallet.dat` or if you have a really old
+legacy wallet it could be at `~/.komodo/HUSH3/wallet.dat` . We will assume the first location.
+
+What we will do is backup your entire `HUSH3` directory, including the blockchain data and wallet,
+then copy the wallet from there into a new directory. This is a non-destructive process that creates
+a new backup of your wallet.
+
+```
+# Make sure your node is not running before doing any of this!
+# Doing this while your node is running could corrupt your wallet.dat
+cd ~/.hush
+mv HUSH3 HUSH3-backup                       # backup all data
+mkdir HUSH3                                 # make a new dir
+cp HUSH3-backup/wallet.dat HUSH3/wallet.dat # copy old wallet to new dir
+```
+
+At this point if you are GUI user using SilentDragon, you can restart the GUI wallet
+and it should perform a fresh sync with your wallet. This will likely take at least
+a few hours or much longer depending on your internet connection.
+
+### On Windows
+
+Basically you want to find where your Hush wallet is, move the directory that contains
+that wallet.dat file to a new name, then create that same directory and then copy wallet.dat into it.
+Different versions of Windows store things in different locations.
+
+For example your wallet might be in `C:\Users\Admin\AppData\Roaming\Hush\HUSH3\wallet.dat` . 
+That means you need to
+
+    * Rename the directory `C:\Users\Admin\AppData\Roaming\Hush\HUSH3` to something like `C:\Users\Admin\AppData\Roaming\Hush\HUSH3-backup`
+    * Create a new directory called `C:\Users\Admin\AppData\Roaming\Hush\HUSH3`
+    * Copy the file `C:\Users\Admin\AppData\Roaming\Hush\HUSH3-backup\wallet.dat` to `C:\Users\Admin\AppData\Roaming\Hush\HUSH3`
+    * Now start the SilentDragon GUI wallet