update README.md to include info on column/muxing

Signed-off-by: Thinh Nguyen <nguyenthinh19011@gmail.com>

Author: tnguy19

src/ram/README.md

@@ -36,6 +36,7 @@ The module will create a new design, therefore a design should not be loaded bef
 generate_ram [-mask_size bits]
              -word_size bits
              -num_words words
+             [-column_mux_ratio ratio]
              [-read_ports count]
              [-storage_cell name]
              [-tristate_cell name]
@@ -58,6 +59,7 @@ generate_ram [-mask_size bits]
 | `-mask_size` | Determines the number of bits which are grouped together for masking during writes. For example, a mask size of `8` will enable each 8 bits of the word to be masked when writing (commonly known as byte masking). A mask size of `1` will enable each bit to be individually masked. The write enable signal for each port will be `(word_size / mask_size)` bits wide. The word size must be a multiple of the mask size. Default: equal to `-word_size`. |
 | `-word_size` | Size of each word in bits. |
 | `-num_words` | Number of words in the array. |
+| `-column_mux_ratio` | Number of words sharing a physical column. Must be `1`, `2`, or `4`. The number of words must be divisible by the column mux ratio. Default: `1` (no column muxing). |
 | `-read_ports` | Number of read ports for the array. Default: 1. |
 | `-storage_cell` | Name of the master to use for the storage device (i.e. a flip-flop). Must be positive-edge triggered. Default: auto-select from the loaded cell library. |
 | `-tristate_cell` | Name of the master to use for the tristate device (i.e. a tristate inverter). It is currently assumed that the device is inverting. Default: auto-select from the loaded cell library. |