title: R merge a particular column from one data frame to another according to its reference column
body: <p>I am a beginner R user.
  I have two huge dataframes and I want to add new column called Vaccine at hkdata.2, which the data is taken from another DF adherence according to 2 reference columns at hkdata.2 (hhID and member), could someone please help me?</p>
  
  <pre><code>hkdata.2
  hhID    member  T0  delta   X_hh    X_fm    ILI age
  1          1    7      0    0       0        0  44
  1          2    7      0    0       0        0  36
  2          1    8      0    1       0        0  39
  2          2    8      0    1       0        0  39
  
  adherence
  hhID member mask soap vaccine
  1      0      1    0    1   
  1      1      1    1    1
  1      2      0    0    1
  2      0      1    0    0
  2      1      0    0    0
  2      2      1    0    1
  </code></pre>
  
  <p>So at the end I can get something like this.
  With an extra column called vaccine in hkdata.2</p>
  
  <pre><code>hkdata.2
  hhID    member  T0  delta   X_hh    X_fm    ILI age vaccine
  1          1    7      0    0       0        0  44    1
  1          2    7      0    0       0        0  36    1
  2          1    8      0    1       0        0  39    0
  2          2    8      0    1       0        0  39    1
  </code></pre>