Add documentation file for overwrite command

[CobbCoding1]

The pull request is simple, it just adds a file which documents the usage of the overwrite command. Similar to the docs/compare.md documentation file.

There is a basic overview of the functionality, basic usage and layout of commands and options, a description of the command, examples of how the command works (with input and output data), an explanation of the SQLite database, and the help message for the command.

[liquidaty]

@CobbCoding1 thank you, could you please make a minor further change as follows to overwrite.md, which is to specify what exact command should be run on th example file, in any place where a command is described?

For example, the below excerpt describes use of bulk-add, but does not provide the actual example command, which presumably is something like zsv overwrite mydata.csv --bulk-add bulkdata.csv. There are a few other similar places that describe commands but need to provide the specific example.

Similarly, instead of writing "table after adding the bulk overwrites above", just append a second command (e.g. zsv select mydata.csv --apply-overwrites) and then show its output (note: just realized that the zsv help does not describe the --apply-overwrites option-- will fix that tonite...). So two commands plus their output can all be put inside a single code-style block that gives a visual of what the terminal would look like with those two commands and the resulting output. Or something like that.

bulk-add would add overwrites to the overwrite file, where bulk-remove would remove matching overwrites from the overwrite file.

Table after adding the bulk overwrites above:
...

[CobbCoding1]

Alright, I will do that

[CobbCoding1]

Should be done now, let me know if there's anything else.