Updated SWGCalculator.xlsx, README.md, and .gitignore

Create .gitignore and ignoring temp excel files.
Updated README.md with correct excel ranges.
SWGCalculator.xlsx:
    Update instructions
    Add description of colored ranges
    Renamed "Min Runtime" to "Max Acceptable" and Max Runtime to Max Possible (24)
    Added setting for SWG % increments, only effects "Calculate SWG %" in bottom left

Author: lightmaster

.gitignore

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+
+~$SWGCalculator.xlsx

README.md

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 Link to TroubleFreePool's forum post: [SWG Run Time Calculator - TFP Forums](https://www.troublefreepool.com/threads/122980-SWG-Run-Time-Calculator "SWG Run Time Calculator")
 
 ## Screenshot
-![SWG Runtime Calculator Screenshot](https://camo.githubusercontent.com/35d0cdd078204d81f96a31e69278459cedbb4109/68747470733a2f2f7332362e706f7374696d672e6f72672f7162346437623078352f5357475f52756e74696d655f43616c63756c61746f725f73637265656e73686f742e706e67)
+[[https://camo.githubusercontent.com/35d0cdd078204d81f96a31e69278459cedbb4109/68747470733a2f2f7332362e706f7374696d672e6f72672f7162346437623078352f5357475f52756e74696d655f43616c63756c61746f725f73637265656e73686f742e706e67|alt=SWG Runtime Calculator Screenshot]]
 
 ## Instructions
-1. Enter your Pool Size in D4
-2. Enter your average 24 hour FC demand in D5
-3. Enter your 24 hour FC output in D6. See [Discount Salt Pool SWG Comparison](https://www.discountsaltpool.com/Compare-Saltwater-Chlorine-Generator-Systems-for-Pools) for common SWG's "Power (lbs/day)".
+1. Enter your Pool Size in D6
+2. Enter your average 24 hour FC demand in D7
+3. Enter your 24 hour FC output in D8. See [Discount Salt Pool SWG Comparison](https://www.discountsaltpool.com/Compare-Saltwater-Chlorine-Generator-Systems-for-Pools) for common SWG's "Power (lbs/day)".
 
-You can adjust the "SWG Output %" to see how many hours you need to run to run your pump. You can type in a desired runtime to see what percent you need to set your SWG to. You can also look at the chart that shows the number of hours your pump needs to run based on percents in 5% increments and an FC demand between 1 and 5.
+You can adjust the "SWG Output %" to see how many hours you need to run to run your pump. You can type in a desired runtime to see what percent you need to set your SWG to. You can also look at the chart that shows the number of hours your pump needs to run based on percents in 5% increments and an FC demand between 1 and 5.