Create README for RA (#120)

Merged in PR #137.

tasks/RA/README.md

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+RA (Risk & Ambiguity task)
+
+-   *High-level task description*: This task consists of “loss” blocks
+    and “gains” blocks. In “loss” blocks, users are asked to choose
+    which loss they’d prefer: a guaranteed loss of $5 (represented as
+    -$5), or a lottery that results in loss of $0 or loss of an amount
+    of money greater in magnitude than $5. In “gains” blocks, users are
+    asked which gain they’d prefer: a guaranteed gain of $5, or a
+    lottery that results in *a gain* of either $0 or some positive
+    amount of money. The choice is displayed for 6 seconds; afterwards,
+    (as indicated by the green dot), the user has 3.5 seconds to submit
+    their choice.
+
+-   *Description of User and Subject Interaction *
+
+    -   **Keys**: \[5/%\] to begin a block; \[1/!\] for the left option
+        and \[2/@\] for the right option.
+
+    -   **Trial duration**
+
+        -   Duration of choice display: exactly 6 seconds
+
+        -   Duration of response window: exactly 3.5 seconds
+
+        -   Duration of feedback duration (after choice is made): 0.5
+            seconds
+
+        -   Duration of ITI: 10 seconds for the initial catch trial in
+            each box; 10 4-second ITI’s, 10 6-second ITI’s, and 10
+            10-second ITI’s randomly distributed across the rest of
+            the block.
+
+    -   **Session duration**: each block has 31 trials; there are 8
+        blocks total (4 blocks x 2 sessions)
+
+-   *Number of trials and values/probabilities involved *
+
+    -   There are 31 trials per block
+
+    -   Probabilities: 0.25, 0.5, 0.75
+
+    -   Ambiguities: 0.24, 0.5, 0.74
+
+    -   Dollar values: $0 (loss only), 5,6, 7, 8, 10, 12, 14, 16, 19,
+        23, 27, 31, 37, 44, 52, 61, 73, 86, 101, 120 (same values
+        negative in the loss domain)