How to display Table in README.md file in Github?

I want to display a table in readme.md file. I read GitHub Flavored Markdown and followed what it said. So this is my table:

| Attempt | #1 | #2 | #3 | #4 | #5 | #6 | #7 | #8 | #9 | #10 | #11 | #12 |
| :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: |
| Seconds | 301 | 283 | 290 | 286 | 289 | 285 | 287 | 287 | 272 | 276 | 269 | 254 |

However, I don't see any table and result looks like:

You need to see documentation again. You can see this cheatsheet

In your case you need to make second line like in example below:

Attempt | #1 | #2 | #3 | #4 | #5 | #6 | #7 | #8 | #9 | #10 | #11
--- | --- | --- | --- |--- |--- |--- |--- |--- |--- |--- |---
Seconds | 301 | 283 | 290 | 286 | 289 | 285 | 287 | 287 | 272 | 276 | 269

difference between this code and your code in repo is that second line with separator has same columns as header. After that this table will be shown

| Attempt | #1 | #2 | #3 | #4 | #5 | #6 | #7 | #8 | #9 | #10 | #11 | #12 |
| :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: |
| Seconds | 301 | 283 | 290 | 286 | 289 | 285 | 287 | 287 | 272 | 276 | 269 | 254 |

Making your example shorter to make easier to understand.

| Attempt | #1 | #2 |
| :---: | :---: | :---: |
| Seconds | 301 | 283 |

And formatted to make it even easier to read.

| Attempt | #1  | #2  |
| :---:   | :-: | :-: |
| Seconds | 301 | 283 |

Headers must be separated by pipe | characters and underlined by - dash characters.

You can create tables by assembling a list of words and dividing them with hyphens - (for the first row), and then separating each column with a pipe |.

First Header  | Second Header
------------- | -------------
Content Cell  | Content Cell
Content Cell  | Content Cell

For aesthetic purposes, you can also add extra pipes on the ends:

| First Header  | Second Header |
| ------------- | ------------- |
| Content Cell  | Content Cell  |
| Content Cell  | Content Cell  |

— GitHub Flavored Markdown

Our example becomes:

| Attempt | #1  | #2  |
| ------- | --- | --- |
| Seconds | 301 | 283 |

Finally, by including colons : within the header row, you can define text to be left-aligned, right-aligned, or center-aligned:

| Left-Aligned  | Center Aligned  | Right Aligned |
| :------------ |:---------------:| -----:|
| col 3 is      | some wordy text | $1600 |
| col 2 is      | centered        |   $12 |
| zebra stripes | are neat        |    $1 |

— GitHub Flavored Markdown

So to center align, our example becomes:

| Attempt | #1  | #2  |
| :-----: | :-: | :-: |
| Seconds | 301 | 283 |